Ha, I've had this happen a few times.
Jonathan Larsson happened to be playing Overwatch, and made a really badass multi-kill as Tracer, the teleporting hellion, that sealed the match. Well, it would have sealed it if he wasn't fighting Mercy, the sole survivor of the onslaught, a support character that has the ability to resurrect nearby players.
So... the whole enemy team came back and won the game. That's rough! But the class based systems that so many titles are adapting these days help make things interesting. Plus, there are counters to double-down on stuff like this that hardcore players will figure out down the line, like focus-firing Mercy.
Teams who generally don't take supports learned a valuable lesson today: