At my old high school, opening night for either of our yearly productions must go off without a hitch, lest we face the wrath of the drama teacher. Last year, for our Follies productions, two people forgot to make their entrances at seperate performances. The first, for the middle and elementary students, the actress was having mic problems and couldn't come out in time. The drama teacher had to run out and make up a conclusion on the spot.
Later, on opening night, the same thing happened in a different scene. Thankfully, the actor (for a high school senior, IMO, he's pretty damn good) can improvise on a dime.
My little sister attended her MS production of Annie Jr. During the scene when Rooster and Lily are introduced, those actors had also missed their cue. The girl playing Miss Hannigan sat at her desk for a minute or two, and then said, "Well, what would YOU do with $50,000?"
However, in all three instances, my drama teacher was apparently livid afterwards.
"During this performance, please feel free to let your cell phones and pagers ring willy-nilly. However, do remember that there are heavily-armed knights on stage and you might well be dragged up and impaled."
(Pre-curtain announcement at the new Broadway musical Monty Python's Spamalot)