Remember, a mistake isn't a mistake if you cover it up well!
I'm sometimes the king of balls-ups, i've just got really good at making them into fills.
Also do not ever stop playing, even if you make a huge gaff, most people won't even notice if you just carry on playing. Scowling at the drummer in an obvious manner helps too
And don't get too p1ssed, we used to get hammered at gigs and it showed