Here's a thread for your greatest, funniest or most embarrassing mistakes, bloopers, blunders and downright stupid decisions you have ever made in a game. I searched for something similar but didn't find any.
I don't know why I have this one so stuck in my memory (remembering about it made me create this thread) but on Deus Ex in one of the final missions I had JC Denton running around on one of the bottom levels and pressing one of those rising platform elevator buttons. I moved him in and waited for the elevator to rise, only to hear the platform noise working but JC wasn't going anywhere. I looked left and right, noticing nothing is moving but the platform's noise is getting louder and louder...
Then the platform came down and squashed me.

Hahah that happened to me in Deus Ex before too.
Myself, I like to throw my character off skyscrapers, buildings, tall places and what not just to see if I can actually "reach" a supposed ledge that's in the distance. I remember when Half-Life was first released, the outdoors skybox was breathtaking. I tried to do a long jump to see if I could land in the water that was in the skybox. No such luck :P

In WoW, my guild leader, another guildie, and I (all 70s) helped another guildie's 50-something alt in Blackrock Depths. At one point in the dwarven city, we cleared to the end of a ledge and jumped off between it and the next ledge to save going the long way through a golem factory.
Guildie: Uh oh.
Guild Leader: Hm?
Guildie: I think I saw the pull on the other side run through the door beside them.
Me: Yeah, I saw that too.
Just then, the entire golem factory charges out of the opening behind us and flattens us in about 2 seconds. Apparently the 50-something aggroed them as he jumped down.

I can't really think of anything off the top of my head, but really, any maneuver I make in a competitive FPS game could probably fall into this category. I'm very much a "fast attack" type of player. The kind of guy who charges into rooms with little regard for his own safety. While this works great for deathmatch and such, it does NOT translate well to games that require one to take a more considered approach. There are times when I'll go like 0 and 10 at Counterstrike simply because I've gone rushing out the second the round starts armed with nothing but a Steyr TMP and a manic grin.
I keep doing it because there are times when it WORKS. Most often, though, it turns out to be an absolute nightmare and ends in my grisly and ignoble death.

I was playing Ghost Mode in Crossfire (MMOFPS) and this team was hiding from us while we stalked them silently with knives...Well, the idiots were yapping about how we would never find them. To their dismay, I was right behind them the entire time and killed them both. The next thing I know I was kicked out of the game because of a 'hacking abuse'. Obviously, the other team was full of poor sports who started to bitch when I killed them...
So, yeah...Never play an online FPS game unless you know the people.