Ouch! He should have just traded Queens and then you can't win with just a Knight and King. But once he made the erm.. error to put it lightly, SHOW NO MERCY of course. If this isn't your 1st instinct then chess isn't your thing mate.
What to do when this happens!?
Sort:
Uhmmm 6 Qa4+ wins for White.
Play the board!
6 Qa4+ (1-0) 
*Shakes fist at xMenace*
Ray_Brooks wrote:
6 Qa4+ (1-0)
You could call it a bad day at the office for Black 
.
Yeah I seen that as well, right after he didn't do it ><, so in a sense, I was happy that I wouldn't lose, but I wanted the win! - Irl mercy is good, but in chess, as atomichicken puts it, show no mercy, just play.
One player being better than another is defined as one making fewer mistakes than the other. This is just a very obvious example. A win is a win. It's only one game, keep playing and learn from this one.
There have been games (with us all) where your opponent OR you have made EXTREME mistakes, in this game I played not five minutes ago, me and my opponent made some extreme mistakes, I knew it was a draw. But then a major mistake ended the game, ive made these before and people did not care, what does everyone think of things like these? Accept draws? Show no mercy??? What?