It happened during previews once when I was doing the show (ASM). It turned out to be something got patched/programmed wrong in the board, though the ME tried to say it had something to do with the fact that a piece of the sign (it was the biggest "Full Monty" sign I've seen yet, three large rolling units since the theatre had limited fly space) didn't get plugged in during the shift. I admit that I gave one of the biggest eye rolls I've ever done and flat out said "So you're telling me that not plugging in part of the sign made front lights come up in the cue?"
I also recall Patrick Wilson talking about the times it would happen during the original run on Broadway. He said once he could just tell that something was off and locked eyes with some mother and her 15 year old daughter right in the front row as if to say "Sorry, this is going to happen" because even if he knew that the effect was not going to go right he still had to do the staging.
