Check out this excellent round-up of disastrous opera performances. My personal favourite is this inspired escape from disaster, in one of my very favourite operas:

As Jezibaba the witch, in the Met’s Rusalka, [Dolora Zajick] had a mechanical cat with glowing eyes and a tail and head that moved back and forth. “While throwing things into the cauldron to make a spell, I would hit the head of the cat with a spoon and sing, ‘Hop, my cat!’ Well, I hit the head too hard, and it fell off, but the tail kept going, and I started to laugh, so I threw it into the cauldron with the rest of the stuff, but the eyes were still glowing! I think the PETA people were upset.”