I was playing Combat Arms.
I was using a wallhack/full bright (******* one), which is a hack to see through walls.
My friends requested it and I sent it to them.
One of them already got me annoyed because he thought he could control me just because he was the clan leader of the clan I was in.
We were playing an Elimination (Deathmatch), and he got all pissy, so, as a joke, I told him that I had edited the .dll to add a keylogger just for him. (He doesn't know anything about programming, lol)
He freaked out, force-powered off his computer, then turn it of again and virus-scan the file.
It found a false positive, because the .dll was packed with Themida (google it/wikipedia it), and he thought it was a real hack.