Lessons learned the hard way

History
Lessons learned the hard way was a huge Rodney entry. It was so large that it had to be removed from the database because Rodney would become unresponsive to everyone else whenever someone requested the entry. For a while it was hosted by GreyKnight, but now that the wiki exists we can put it where it really belongs.

The Lessons

 * Don't try cancel your own spellcasting by pressing Escape, especially when polypiling. Skipping the direction prompt releases the magic power to the previous direction!
 * When you get a blessed scroll of genocide, @ means you, even though you're a gnome.
 * Forcing the lock on the chest with a blunt weapon on the castle level isn't the best idea.
 * Sleep rays bounce, and elves don't start with sleep resistance.
 * Beware the hallucinating pet arch-lich. (Make sure that that black light over there blows up in your face and not your pet's.)
 * "huh. Didn't know a bag of tricks would explode my bag of holding."
 * You have to really be INSIDE a temple for sanctuary to work. The door isn't good enough, unless the monsters are inside the temple too.
 * Don't eat wraith corpses when satiated. You don't get a warning.
 * Don't wear levitation items until you need them. They deprive you of Elbereth and other useful escapes.
 * If you just spent a year extincting/genociding everything and then polypiling gems, hauling them to the first floor, trying for hours to get your giants arranged around the upstair, and Rodney comes...for the love of God, don't quaff the potion of gain level he may drop on death without checking B/U/C first. Lives have been ruined that way.
 * Levitation spells don't cut it on Medusa's Island. Find a permanent source or get skilled.
 * Be wary of statues in the Gnomish Mines...
 * Don't bring a pet gremlin to Medusa's level.
 * Don't panic. Always know where your towel is.
 * Don't EVER tangle with Cyclops, the Healer quest nemesis, in melee. MC3 doesn't help.
 * Be very careful with uncontrolled teleportitis in stores! Drop your gold first.
 * Plains centaurs in the mirror are stronger than they appear.
 * Baby silver dragons don't reflect zaps of cold that you shoot at them. Looks like it's going to storm!
 * Hitting e instead of w can be a fatal mistake when you have a cockatrice corpse.
 * Just because you beat Sokoban and got a bag of holding, it doesn't mean you're invincible.
 * Remember, "Thou shalt not worship graven images"
 * Never sacrifice something you can't touch! Attempting to lure a cockatrice onto an altar for sacrifice with the idea that you can do so without touching it will cause you to develop a stony appearance without any chance of eating a lizard
 * Never eat tins in shops whilst wielding potentially expensive artifacts.
 * Using waterwalking boots on the plane of water will result in you drowning. Levitation, on the other hand, will work safely.
 * Always use up cockatrice eggs you lay. Tame cockatrices turn any golem to stone golem, which is hard to kill.
 * Cursed athames aren't good items to engrave with.
 * Don't carry wands/scrolls/potions outside bags. Nothing like standing on the vibrating square with the Bell of Opening (0:0), complaining about exploding wand of cancellation and wand of wishing and not having a scroll of charging handy.
 * Don't rely on your quest artifact as the only source of ANYTHING.
 * Don't stand on ice when there are foes with fire attack around.
 * Don't try to disarm/disarmor your Archon with F command when polymorphed into a nymph. You can untame it.
 * If you have crappy ISP, NEVER reconnect to server right after you got your internet connection back. There can still be some glitches with it.
 * NetHack doesn't know about arrows/function/home/end/page up/page down keys and treats them as escape. Therefore DON'T TRY TO EDIT YOUR WISHES WITH THEM.
 * NEVER have potion stack directly on altar when there are intelligent foes. Wand of striking == BOOM!
 * NEVER skip messages with space. Ever. Especially when handling petrifying monsters.
 * What do you think amulet of life saving is for?
 * When polypiling, be careful with where you aim. In particular, don't hit that damn chest with goodies - if it polymorphs into non-container, ALL CONTENTS ARE LOST.
 * When you have laggy connection, NEVER type next command before you see how previous one ended.
 * When running from a nasty monster, go back the way you came.
 * Be careful of what items are marked as 'y' and 'n' in your inventory to avoid confusion with yes/no prompts. For example, trying to eat something on the ground while levitating by typing 'ey' quickly will have disasterous effects if your inventory contains 'y - a cockatrice corpse'.
 * Be careful of what items are marked as direction-moving keys in your inventory, in case you miss a key, so instead of 'ahky' (apply h-a key north Unlock the door? yes) you hit 'aky' (apply k-a wand of cancellation Really break your wand of cancellation? yes)
 * When hiding in a shop to recover HP, make sure to rest instead of search to avoid finding (and waking) mimics.
 * When breaking wands for its area-denial purpose, DO NOT break a wand of lightning unless you hate your rings. Especially not when Demogorgon is on the level. Not even if double-meleeing arch liches.
 * Make sure comestibles (esp. rubber chickens) aren't assigned 'e', wands aren't assigned 'z' or 'a', potions aren't assigned 'q', scrolls and spellbooks aren't assigned 'r', tools aren't assigned 'a', weapons that might be cursed aren't assigned 'w', armor that might be cursed isn't assigned 'W', rings that might be cursed aren't assigned 'P', etc.
 * Wield the cockatrice corpse when you're already in position to attack (you won't be moving into any unknown squares). Otherwise you will fall into a pit or spiked pit on your way to your victim and be stoned instantly.
 * When the ground around you looks the same except for being a different color, examine it with / or ; before stepping there.
 * Don't anger your god after getting divine protection. Praying for unholy water on a crossaligned altar, for example.
 * Don't go below dungeon level 18 or so without a means of crossing water. Falling past Medusa through a hole can really ruin your whole day.
 * Don't melee cockatrices if you're polymorphed, especially if you are a vrock, master mind flayer, vampire lord, dragon or any other biting monster.
 * Don't try to polymorph a merely annoying monster just to get it out of your way; save your charges (or mana) for things that are just about to kill you; the creature it becomes might be something deadly. (StormEagle, blue jelly =&gt; ice devil)
 * Drop your gold, bag of holding, and unicorn horn on another square before ting on a throne.
 * If you find Minetown bones, before you go #chatting up the priest, make SURE both the priest and the altar are of the same alignment and the temple isn't haunted.
 * If you have no gloves and are blind and have recently dispatched a cockatrice, always move with the 'm' key. Doing so will cause you not to automatically feel objects on the ground.
 * If you levelport, angry shopkeepers are somehow able to levelport with you.
 * There is a vital difference between '&lt;god&gt; is displeased' due to praying too soon, and praying on the wrong altar or with negative luck/alignment. Only the first one makes your god angry.
 * Any monster in the 'follower' category will follow you through portals created by the Eye of the Aethiopica.
 * If you are being held/crushed by an owlbear, zapping down with a wand of digging is not as effective as you might think. Engrave Elbereth with it instead...
 * If you leave skeleton keys (but not lock picks or credit cards) lying around, monsters that can open doors are smart enough to pick them up and unlock doors with them.
 * If a fountain in Minetown reduces to a trickle, cease using it immediately.
 * For beginners: if your HP shows as yellow or red (with the HPcolors patch), you have the spell of healing and you have enough power to cast it, do so. You'll never know which gnome has the wand of lightning/cold/fire/death.
 * If someone throws an egg at you, it might have been a cockatrice egg - don't skip through the messages about stiffening and turning to stone as if they were ordinary battle spam, if you don't want to become a statue.
 * If you don't have a way to protect against touch of death, NEVER step off of an E-square when facing an arch-lich.
 * If you're trying for #conduct weaponless, just leave pick-axes on the ground. Don't even bother to put them in your BoH. Use wands of digging instead (which are not wielded). If you're going for #conduct pacifist, wield a cream pie and never let go. Rely on 5 to move.
 * Never underestimate a bones level.
 * While trying to tame something using charm monster that can resist, you'll probably gain a lot of pets. Naming them *all* will help keep you from sacrificing a pet (since corpses retain names) and losing your 50 points of protection.
 * Get a second or third opinion. ALWAYS.
 * Don't eat anything while standing on Medusa's corpse. You WILL fatfinger it and suffer a legendarily stupid stoning.
 * Always kill shapechangers when you see them. If you don't they turn into bad things that will kill you.
 * argiopeweb -- Minotaur in Soko3
 * Demin - Black Dragon Gnomish Mines lv 2
 * RegalStar - Archlich in Soko4
 * Don't stone-to-flesh statues from a bones file. They are likely to be well-armed and armored.
 * If a human zombie dies on your altar during a sacfest, move the human corpse away before trying to sacrifice anything.
 * If you see any messages pertaining to a swamp, do NOT blithely skip along the level. You WILL hit the water, and all your stuff WILL get blanked.
 * Monsters will follow you through your quest portal. Engraving on the portal square doesn't work.
 * If you're dipping potions into a fountain, don't typo and dip your bag of holding. The RNG will curse it. Every time.
 * Wielding a cockatrice corpse around a foocubus is an excellent way to get a permanent hard-on.
 * Don't try to feed a dragon unless it is currently tame. If it is peaceful, thrown food will anger it.
 * Zapping a high priest/priestess on the Astral Plane with a wand of teleport in order to move them off an altar DOES NOT WORK. It makes them angry instead, even if they were peaceful beforehand.
 * Don't just try on unidentified rings if you're near a priest or any other friendly. If that ring turns out to be conflict, you won't live to regret it.
 * Locking a door will not do much if you leave a pick axe or mattock immediately outside it.
 * Never, ever, ever jump near your quest leader. Your quest portal will be removed, with only one way to salvage your game: the Eye of the Aethiopica.  Better hope you're a neutral non-wizard.  It also helps to be able to cast charm monster with a relatively low failure rate.
 * When in Sokoban, and you are at the fringe of death, do not try to teleport away using a scroll of teleportation. Sokoban is a no-teleport area, which will result in a few more hits from that monster you are trying to flee from, possibly resulting in your death.
 * When you solve a Sokoban level and open the door at the end of the hall, it will be packed with monsters. One might just be a Nymph who will charm you, steal your Wand of death, fail to teleport then use it on you.
 * When attacking a gas spore, make sure your pet isn't within the range of the explosion when the spore dies. It will count as if you killed the pet. This also applies to aligned Priests - losing the only co-aligned temple in your entire dungeon is a real bummer.
 * If you want to create Excalibur in Minetown, make absolutely sure that there are no guard around (or that you have a pet strong enough to deal with them). Even if they aren't in sight, they will somehow sense you and get angry at you, and killing them count as murder.
 * Don't try to polymorph at experience level 1 or 2, unless you like being killed by unsuccessful polymorphes.
 * Don't zap your kitten with a wand of polymorph unless you're prepared to risk the consequences of killing it yourself (System shock).
 * If you genocide giants before you do the Valkyrie quest, you will get lots of titans instead. You don't want lots of titans.
 * Before you kick in a locked door, make sure it's not attached to a shop closed for inventory, lest you anger the shopkeeper.
 * In Moloch's Sanctum, be sure that you have entered the temple, and that the high priest has called you an infidel, before killing him. If you don't, you'll get dinged for murder.
 * Don't eat meatballs while satiated, you won't get a warning!
 * Don't polymorph in &gt; direction while riding. Even your ki-rin is not invincible, A's are vulnerable to system shocks!
 * Watch where you kill the Wizard the first time. Killing him above the moat is a VERY VERY bad idea if you can't swim and don't have a wand of cold (the book will appear above the ice!).
 * When waiting for monsters to show up and force you out of were-form, don't wander away from your only weapon just because you couldn't carry it. Or throw junk upwards.
 * After you're done dancing with nurses or foocubuses, DON'T FORGET TO RE-EQUIP YOURSELF!
 * On that note, be sure to remove all armor AND weapons before reverse-genociding that swarm of nurses.
 * A xorn will gladly munch your Magicbane, especially when you disrobe to take a bath and blank scrolls.
 * When you're on on a level with water or lava, and you have uncontrolled teleportitis, and your connection is laggy, TREAD LIGHTLY.
 * If you've altar scummed for Magicbane as a wizard who started with a ring of slow digestion, don't forget that you can actually use it to engrave an E-square when you're facing giant ants
 * Be careful about wishing for cross-aligned/cross-class artifacts when low-level or low on health, especially when said artifacts are intelligent.
 * When playing with number_pad off, be careful when going northwest. "Really attack Izchak?"
 * When in a zoo, fight shriekers and gas spores last.
 * Standing on a burned Elbereth and reverse genociding dragons to create DSM is a great idea... unless you have teleportitis.
 * Pause every time you see a monster. Watching the contents of your inventory, or what you are polymorphed into, is free. Thinking of a tactic is also free. Going toe-to-toe with a footrice when you're poly'd into a master mind flayer is priceless.
 * After spending hours of hard work and effort to descend into Gehemmon, only to be swamped by monsters, remember to check your status as often as your inventory. Scrolls of genocide may be great escape items, but only if that umber hulk next to you isn't looking your way! (And no, your AoLS will not save your sorry self here!)
 * Werecreatures are considered human. Sacrificing werecreatures while not chaotic is not a good idea.
 * AVOID foocubi of opposite gender when levitating over water in Medusa's level using boots. Drowning is not kinky.
 * Chameleons leave brown corpses. They can also impersonate monsters which leave brown corpses.
 * If you drop your weapons and armor to go for a scroll-blanking swim in the castle's moat, please take care where you leave those items lying around. Chasing down a master mind flayer that's made off with your +7 Grayswandir is not fun when you're unarmed.
 * When you get to the castle level, do not burn Elbereth on the drawbridge and fight the monsters there. A wand of striking will kill you instantly, and it won't be fun.
 * Even if you've been struggling with containerless inventory management for 10,000 turns, and you've finally got a bag of holding from a wish, don't get all excited and just throw everything into it. That pesky wand of cancellation will go in last, and then you're screwed.
 * If your luck is negative, or even if you think it MIGHT be negative, DON'T WISH UNTIL YOU FIX IT!
 * If you're confused in a shop and standing next to the shopkeeper with your very promising character, keep doing nothing for as long as it takes. Yes I know it's only a 1 in 8 chance. Yes it will kill you.
 * Plugging the trap doors on the castle level with boulders is a Bad Idea. It will result in the lower levels being inaccessible, except through a teleport-controlled level teleport; you cannot dig downwards on the castle level.
 * A bugle is not a good tool for a knight to use to wake sleeping monsters - especially in the mine town.
 * Never use force bolt in a level with a shop (you hear the chime of a cash register) if you haven't explored the area in the direction you're shooting if you can't pay for broken things, unless you can fight the shopkeeper.
 * Never eat an unknown tin whilst hallucinating. It WILL be cockatrice meat.
 * Engraving with wands is useful. Unless you get the message "The bugs on the floor stop moving!" More often than not, it will be a wand of sleep, but rarely it can be a wand of death. Don't zap it on walls that can reflect back to you (all of them). It will be that wand of death.
 * If Branchport into Fort Ludios, you may very well land in the area just outside the inner keep and get swarmed by everything. While it's (generally) not a fatal error, it can still get rather annoying when you can't deal with about fifty creatures one-on-one.