Most MMOs: There are a lot of stupid people - if you can't understand simple instructions (kill the healer, don't aggro the overpowered boss, etc), please stop eating. And drinking. And breathing.
RTS games: If you have artillery, pat yourself on the back - you've just won.
Online RTS games: "gl hf" is a sign of impending doom.
Morrowind: No matter how many enemies are chasing you, just close the door.

Everygame with Auto Healing: if you wait long enough, you can recharge back to full health from a shotgun blast to the face.
Games with Medkits: If you use a medkit, you can survive a direct hit from a rocket and be able to shrug it off.
Quake 2 era games: Tapping jump will make you go faster. Bunny hopping is the sign of a true pro.
Quake era games: Rocket jumping is a great skill but grenade + rocket jumping is pro.

Street Fighter 4: The more pissed you are the more powerful you become. To the point that you can beat your opponent multiple times in a very cool way.
Ultima, Fallout 3 and etc: No matter how much food / alcohol / cola / etc you consume you'll never need to take a dump or a piss.

Mount and Blade: William Wallace in Braveheart had the best method of killing horses, with a sword as tall as a man you cleave the legs! And all my men have the spines of gummy bears!

Fallout 1: You really should FIRST learn how to drop items from your hands/inventory before use dynamite with 20 sec timer.

Heheh, it was more like 'suicide by accident'. I just didn't get it.
After that I learnt it quickly.

How to manage my time. :D
Yes, yes of course I'm joking!

Baldur's gate : Learn how to hate Imoen and how you to turn of NPCs voices.
Baldur's gate II : Admit that you where wrong about Imoen she is actually a nice character.
Mount & Blade : Playing the big guy without a shield is seriously the most stupid idea you had, standing in front of charging knights too.
Vampire Bloodlines : Never trust anyone.

Splinter Cell: Standing in pitch darkness in no way emphasizes the bright glow of the various instruments you're carrying to people who are trying to look for you.
Splinter Cell +1: If you have enough upper-body strength, you can do some seriously crazy stuff.
Splinter Cell +2: Tasers are more effective than bullets.
Splinter Cell +3: You will never encounter anyone who values his mission over his life; only you and possibly the main villain get to play THAT role.
Splinter Cell: Chaos Theory: Being killed by a ninja is an awesome way to die.

Tom Clancy's Ghost Recon: The heaviest armor in the world won't save you if you don't have the element of surprise and, ideally, superior firepower.
Deus Ex: Tear gas, Tasers, and Tranqs are the Takedown Trinity. Awesome alliterative appeal!
Phoenix Wright: Ace Attorney: If you're a defense lawyer, you can totally steal random crap from the scene of a crime and use it not only to prove that your client is innocent, but that the witnesses, police, or even the prosecution are guilty. Usually, no one will complain about your blatant violations of evidence law.
Phoenix Wright: Ace Attorney again: If you're a prosecuting attorney, you can not only steal random evidence from the scene of a crime without telling the police, but you can also deliberately conceal said evidence as well as anything the police collected and possibly get away with murder (literally). As long as you don't actually FORGE any evidence, you're fine, and even if you do forge evidence you're okay if no one calls you on it.
Phoenix Wright: AA+1: There are no honest prosecutors, only people who are driven by insanity or tragic pasts to knowingly and willingly convict innocent people.
Phoenix Wright: AA+2: Every defense lawyer needs a quirky junior sidekick to hinder his or her every effort and jeopardize the trial.
Phoenix Wright: Ace Attorney (1st game): When you have uncovered the truth about who really committed the murder your client got blamed for, the first thing you should do is run straight to that person and accuse them of murder, to their face, out of court, without alerting the police first or getting a subpoena. There will be no serious negative repercussions for waving your finger in the face of a known killer in a place where there aren't any witnesses.
Phoenix Wright: Ace Attorney: Justice for All: Disregard PW: AA1. Confronting murderers with accusations of murder may very well get you murdered. Obviously.
Phoenix Wright: Ace Attorney: Justice for All again: There's nothing sadder than watching a mentally disabled person commit suicide.

Super Smash Brothers Brawl: Speech is in no way necessary to communicate with other people. Grunting, making squeaky noises, or outright beating the pulp out of whoever you're attempting to connect with are all perfectly acceptable/practical ways of conveying meaning to others.

Back in the old days I learn this with (oldskool tabletop) Blood Bowl.
Although it's always polite to be sorry for opponent's best player who died by your fault acts while he was already down.

Actually, nope. Bolter rounds are self-propelled bullets, but they are still bullets, and even thou their armor piercing abilities are better than thouse of "normal" weapons. Yes, they do have explosive cap, but explosive that rips infatry to pieces doesnt necesarrily have to be enough for heavily-armored vehicles.
On the other hand, thunder hammers are designed to pierce trough armor, they are even designed to discharge a power surge into their target, disabling its electronics.
See? Thats how you know Im playing warhammer way too much again. Part of me is still somewhere in 41k universe, shooting orks. I blame winter

Any FPS Game: There are a lot of people in the world whose only function is to be killed by you in order to demonstrate how awesome you are.
Any FPS game in which special armor is worn by the protagonists: Armor is all it takes to turn an average guy into a super-soldier.
Any FPS game in which special armor is NOT worn by the protagonists: Armor is useless.
Any FPS game that involves psychic/magical powers: People who are not superheroes exist primarily to be the pawns of superheroes, living or dying at their whim. The Special Ones are the Masters, whether we choose to admit it or not.
Most FPS games: If bullets don't work, use more of them. If that doesn't work either, try progressively larger explosions.
I'm raising this thread!

Fleet Defender: If you try to pull off Maverick's move from Top Gun, YOU WILL DIE.
Jane's Fighters Anthology: Verified that Maverick's move in Top Gun is very very stupid.
Il2-Sturmovik 1946: A P-47 Thunderbolt can chop off a damaged wing of a BF-109 without being damaged itself. There's a video of this too. Just google "Il-2 lucky blunder"
Test Drive Unlimited: The cops in Hawaii don't chase you if you drive past at 300km/h.
Rise Of Nations and Empire Earth 2: People with swords and chop up tanks.
S.T.A.L.K.E.R.: Vodka cures radiation poisoning.
F-15 Strike Eagle III: If your navigator really hates your flying he/she can swap seats with you in mid-air.
T.F.X.: The world is full of lizards.