The biggest, most "WTF?!" Moment I've ever had, was when my friend and I were playing Republic Commando on the original Xbox.
We actually got this to be a hosted glitch on a few websites.
Basically, after we did this random odd amount of things, we loaded up a single player save. For whatever reason, we had no one in our squad, except for Scorch, and a number infinity sign. However, we only saw 62 at the start. We proceeded to walk up a ramp, and out of nowhere there was a random Geonosian standing over a regular, but very white and shiny, clone, who was down. We killed the bug, and then revived the clone, but then he just started hovering. Afterward, we couldn't complete the game.
If you let 62 heal him, you can't complete it either. And if you try to kill the clone, 62 will start attacking you.
Most random thing, ever.