I was once killed by someone using some kind of PvP bot in WoW. How could I know?
When a level 69 Tank with an EPIC AXE is killed by a level 64 Night Elf at half health, there's something going on. I'm not saying it's impossible, but it pretty much looked liked he was using the cheap jumping around and glitching trick.
This happened yesterday, in fact - I was playing 2Fort in TF2, and one of the red engineers was dressed in blue. He only appeared in red after he was killed, and it took our team a while to realize the foul play. Still, I was discriminated against (I was also an engineer) and everyone was suspicious of everyone.