Rocket jumping in Team Fortress 2: Gain tactical advantage, sometimes die.
Rocket jumping in Borderlands: Lose health, sometimes die.
Strafe while running in 007 Goldeneye/Perfect Dark: Get speed boost, complete missions faster.
Strafe while running in Far Cry: No speed boost, lose mission because you can't see shit.
Snaking in Mario Kart DS/Wii: Massive speed boost when done properly.
Snaking in Trackmania United: Crash into a dozen walls.
Press Q to switch weapons in Half Life 2: Fast switch to a better weapon for your situation.
Press Q to switch tools in Minecraft: Toss that fancy diamond picaxe into some lava.
Bunny hop in Portal: Move faster, solve puzzles easier.
Bunny hop in Counter Strike: Source: Move slower, die easier.
Shoot rocket at wall next to you in Red Faction: Instant cover, sometimes shortcuts.
Shoot rocket at wall next to you in Half Life: Lose health, sometimes die.
Using VATS in Fallout 3: Easy damage, usually perfect accuracy.
Using VATS in Left 4 Dead: Easy death, usually takes a crane to get it out.