I was playing my trumpet in an orchestral setting. The piece was Ravel's "Bolero" and for some reason known but to God the whole damned band, Director and all, got lost. We were playing the "dum diddly dum diddly dum diddly dum dum" part and we kept repeating the same notes over and over. Finally the first trumpet realized what was going on and with a fortissimo blast began the next section -- everyone woke up and continued as if nothing had happened.

Oddily enough, the audience never realized what had happened!