Ancient topic, but I hope nobody minds if I add my two cents...
I've seen a spectacular one, unfortunately. It happened during the English National Ballet's tour in 1999, at the Sydney Entertainment Centre (which admittedly does have a notoriously slippery stage). They were doing a fairly standard Swan Lake, but doubled in size to make it an "Arena Production" -- a terrible concept that turns ballet into a circus. I can't remember who danced Odette/Odile on the night, but after a handful of fouettes, she lost her centre and fell over. She stayed on the floor for a while, then struggled back up and finished the remaining couple of fouettes, switching to her working leg.
It might have been the arena setup, or just the size of the place, but it made for a pretty dsimal experience overall (not just the fall, the entire performance). I admit I've been prejudiced against the ENB ever since...