.. Mine has to be when I unlocked Cyrax. I was messing about with his bombs and talking to my bro while I was doing it. I was playing against the computer who was Scorpian. He was about 2 jumps away from me and I fired the close range bomb.
As the bomb was about to go off I laughed and said 'Oh come on, theres no way that thing will get anyone'. At that moment, Scorpian jumped ontop of it and set it off anyway.