I've noticed I dodge my head to avoid incoming fire. It happens more when I'm low on health, especially when playing Doom. I also noticed, after it was mentioned that I tilt my head slightly in games that allow me to lean around corners. When I'm sniping in games, or even just scanning an open area using iron sights I tend to get tense and lean in closer to the screen. Hell, in Soldier of Fortune 2 I'm sure I've been 6 inches away from my screen straining my eyes without realizing it.
When I was a young'in I would always jerk the controller up drastically when I jumped while playing Mario Brothers. I can't count how many times I pulled the controller out of the NES, and consequently died as a result.