I was lucky to witness the final performance of August: Osage County where Rondi Reed was called in to play Mattie Fae when Elizabeth Ashley had to call out due to illness.
Marin Mazzie was a replacement understudy in the original production of Into The Woods. As I remember it, only about a week after she signed her contract, she had to go on for The Witch. She had been through music rehearsals and some blocking rehearsals, but she had to go on. She said she felt like she had been "shot out of a cannon" after it.
I did a regional production of Les Miserables where our Valjean fell and broke his ankle BAD only about 10 min into the first show on a two show day. We had to cancel the show, and as soon as the paramedics put him in the ambulance we immediately started a put in rehearsal for the understudy who then played the role that night. I think we had an hour off for dinner after the rehearsal was finally over. I was sitting next to the understudy when Valjean went down, and frankly I just don't know how to describe the color he turned.
"If this is going to be a Christian nation that doesn't help the poor, either we have to pretend that Jesus was just as selfish as we are, or we've got to acknowledge that He commanded us to love the poor and serve the needy without condition and then admit that we just don't want to do it." -Stephen Colbert