Victoria Clark was out, famously, the Sunday before they recorded the cast album (the day I saw the show!). When I did a tirade about it, I was put in my place at this board and another. She was apparently really ill, not saving her voice or some such. And as far as I know, she hasn't missed since. I must report again, Patti Cohenhour was stellar, moving, accomplished, and seemingly very rehearsed, when apparently she was not that infamous Sunday.
For my money, the person you don't want to miss is Ms. O'Hara. I thought she was the musical centerpiece of the show. But of course, the day I saw her, Mama was out.
"I'm a comedian, but in my spare time, things bother me." Garry Shandling