I've had "brain cramps" on stage before. Sometimes you just forget a part of the song. A couple of the things I do, is to hangout in E or A until I snap out of it. It only lasts about 4 or 5 seconds, but It's long enough to look back and see your drummer laughing at you. Once I forgot the bridge to one of our songs, but it was remniscent of "Crosstown Traffic", so I played that until I caught up. Afterwards, the lead singer said, "That was brilliant. What made you come up with that?"