God, I must have done that countless times. In Republic Commando, I would often find my squadmates stuck on things, but I could usually get them out again by killing them then reviving them. But one time, he wouldn't come unstuck and eventually the squad all started shooting at me for my constant killing and reviving of the guy who was stuck. But then, that wasn't really my fault so let's move on.
In Garry's Mod, I downloaded a few maps but whenever I opened them, almost everything was replaced with big red "ERROR"s and when I looked around, the sky changed to... I'm not sure how to describe it. Anyway, turned out I needed CS:S for those maps to work so I bought CS:S during the weekend "CS:S for £3.50" sale on Steam that week.
On the subject of CS:S, I somehow managed to get a BSOD from reloading in CS:S. I imagine that the two things weren't entirely connected but I'm sure it at least had something to do with the fact that I was playing CS:S.
Oh and before I forget, I once managed to cause the pistol in HL2 to be able to reload even when on full ammo. I can't remember how, though. :S