When we see the show from the stage, there are fake walls visible behind all the doors. When we see the show from backstage, those fake walls are missing from all the upstairs doors (although present for the downstairs doors).
During the Des Moines performance, after Gary comes down the stairs saying, "Sex criminals everywhere! Vicki? Vick?" he kicks the tin that Belinda had dropped earlier. The one he kicks is very clearly dented by his kick, but the original tin lies upstage near the door to the kitchen.
In the Miami matinee, after Freddie's quick change, he trips and falls and both his headband and glasses fall off before he knocks on the door and falls through. In the next shot, his headband and glasses are back in place.