Brent Barrett was the standby for The Baron in GRAND HOTEL when he performed "We'll Take a Glass Together" with Michael Jeter on the Tony Awards. Rex Smith was in rehearsal for the role at the time, and the producers didn't feel he was ready when the televised ceremony rolled around. Barrett has spoken publicly about how awkward it was to have such a big triumph and then be standing in the wings watching another actor perform the role the next night.
Subsequently, Smith left the show shortly after he started, and Barrett assumed the role on a permanent basis (I think he also did the road company.)
Tonya Pinkins: Then we had a "Lot's Wife" last June that was my personal favorite. I'm still trying to get them to let me sing it at some performance where we get to sing an excerpt that's gone.
Tony Kushner: You can sing it at my funeral.