I feel genuinely bad for the person this happened to.
Imagine this: one second, you're creeping around corners, scanning your surroundings for any hint of movement, any telltale sign of life looking to stop you cold. A flashbang goes off, and you use the momentary distraction to scamper to a perfect position.
Everything is going well for you, the esteemed Mr. AceKillFace. You feel good about yourself and your talents as a Counter-Strike player. Your ex was wrong about you. Your dog ran away for unrelated reasons.
Then someone literally climbs on top of your head to boost themselves above cover, kills your teammate, kills you, kills another of your teammates, and wins the game.
"That just happened," he says immediately afterward. "That's just happened."
So yeah, let that be a lesson to you: always watch your back. And maybe just grow eyes in the back of your head, if you can.