Happens to the best of the best. On my tour we had 2 cast members go down for our production of R+J which triggered a series of covers which brought our company manager onto the stage and into an equity contract for the week. It was an exciting performance to say the least.
My favorite part about those performances is seeing the cast's interest resparked in the show. After 50 performances in 20 cities attention spans wane and they spent their off time in dressing rooms/green rooms. Almost no one left the deck. They wanted to be there to get the new feel of the show and support their cast mates as they plunged into new tracks.