Revealing mistake: During the Inquisition song when Right Before Mel Brooks Plays bongos on the guys heads he slides in from behind the wooden torture things. If you look closely the hair on the stunt double sliding in is darker than Mel's.
Revealing mistake: When Betty and Oliver go to the coffee stand, immediately after Betty says "Plus one," you can see that the guy working the stand just grabs two cups from his stand without pouring coffee into them and gives it to Betty and Oliver, who drink from the empty cups.