After playing certain games some things happen:
I try to reload in Mass Effect.(both with X and RB)
I try to use VATS in Gears of War.
I'm so used to Half Life 2 with SMOD that I reach for mouse4 to slow down and mouse5 to enable ironsights whenever I see an enemy.
Fallout3, well, I press B to melee, try to enable bullet time when playing with the keyboard(always on Z for me) and I try to throw grenades with mouse3(my default for any game where grenades have a special button).
EDIT: they're not the only games it happens in, but most common since I play those a lot.