Mistakes and anachronisms

At the very beginning, the Zac Ephron character shows that he can play a double-stroke roll on the snare drum. However, like most very young people today, he uses a matched grip, which would have been unthinkable for any drummer in 1937. The first time the matched grip came into general use (apart from tom-tom solos like Gene Krupa's on Sing, Sing, Sing) was when Ringo Starr (who is naturally left-handed but plays right-handed) used it. Certainly, it would have looked exceedingly odd to any musician in 1937.

Also, when everyone is waiting for Orson to show up, one of the stagehands responds to the question "Is Orson on his way," with the comment, "Hopefully, he is." No one would have misused the word "hopefully" like that in 1937 -- that kind of stupidity had to wait for the 90s.