It's happened to me on several occasions.
First, back in January of 1999 I went to see the
Cabaret revival and couldn't wait to see Alan Cumming. It turned out to be a very odd experience. It was the Sunday matinee, and although it was only 10 minutes before curtain they weren't letting anyone in. It had been raining pretty hard that day, and there was water leaking into Studio 54. The opening was originally delayed due to clean up. Then about 15 minutes after the show was supposed to start, an usher came into the lobby an announced that Alan Cumming had left the theatre sick. Since it happened so sudden, no one was ready to go on, so the show would be delayed an additional 30 minutes so Vance Avery could get into costume and make-up. The show ended up being great and Vance did a wonderful job on such short notice.
A few others: I missed Raul Esparza in
Tick Tick Boom the first time I saw the show; I missed Stokes AND Marin Mazzie in
Man of LaMancha; and I missed Norbert in
Dirty Rotten Scoundrels. Although I only make one trip to the city a year (so I only get one chance to see each show), I still had a great time seeing each of these shows with their understudies.
"You pile up enough tomorrows, and you'll find you are left with nothing but a lot of empty yesterdays. I don't know about you, but I'd like to make today worth remembering." --Harold Hill from The Music Man