My best moment so far has to be one time I knifed 4 people from behind on the upper hallway on Pump Jack, but that one wasn't the most hilarious one.
The most hilarious one is actually me getting killed but in a very WTF way, on Snow valley, I was running behind an enemy sniper who didn't notice me, with a knife. I was about 2 meters away, ready to stab, and then , suddenly, I got HS'ed by a *friendly* sniper, in this FriendlyFire-OFF room, THROUGH the enemy's body, who didn't die from the shot, but shot back and killed my teammate after that.
The only possible explanation I can think of is that the game only decides the damage mode of a bullet when it hits someone and nowhere after, so when it hit the enemy it became a full-damage bullet, and applied to the teammate behind it too: me :|