Tova Feldshuh didn't have an understudy or stand-by in Golda's Balcony, so if she ever had to miss, the show probably closed and made up the performance at a later date.
I know that Antonio Banderas never missed a performance of Nine, and his understudy didn't go on until John Stamos took over. How do these guys do it? They go on even when they're feeling like crap on a cracker and get cortisone shots if their voices are gone. That's serious dedication.
"Peace! The charm's wound up."
--Macbeth