Factual error: When The Oneders are recording their song with Uncle Bob, there is an In-N-Out cup sitting by the recording equipment. There are no In-N-Out restaurants in PA.
Factual error: When young David and his father show up at Ben Rosen's house, there is a Jewish mezuzah (a handwritten tiny scroll of parchment with a passage from the Bible enclosed in a case) affixed to the doorpost. The position of the mezuzah is incorrect. It should be higher up on the door jamb and the top should be angled towards the house, not the other way round. (00:15:00)
Factual error: When the band are playing "March of the Cobblers" at Saddleworth, Andy (tenor horn) and Harry (euphonium) are heard playing the main tune of the march. In real life however, these two instruments do not play the tune, they have separate parts for the accompaniment.