During "That's Rich", Jack can be seen on the left of the stage as a member of the audience watching. At about 27:44, a different man is sitting in Jeremy Jordan's seat, and it's clearly a different person. Within a few seconds, Jeremy Jordan is back in the seat. It has been reported that while one live performance was given for the taping, the show was also performed multiple times over several days to an empty theater for filming as well. Likely, multiple takes of the performance were edited into the final film and Jeremy Jordan must have needed a stand-in for at least one take.
Jack changes the hand he points at Pulitzer with from left to right, when asking which trait gives them the most in common.
At 1:36:19 during "Brooklyn's Here", look to the bottom left of the screen and it's obvious that it's not Jeremy Jordan on the table being wheeled out.
When the children's crusade newspaper is projected on to the set, Jack Kelly is misspelled as Jack Kelley.
Race (Ben Cook) accidentally kicks himself in the face during "Seize the Day."
Specs (Jordan Samuels) bails out of a cartwheel during "Seize the Day" after getting accidentally kicked in the mouth.