When Teddy is playing the harpsichord near the beginning of the film (c. 6'45") the music on the stand is a duet for an instrumentalist with a keyboard accompaniment, hence the three music staves, yet we hear Wolfgang Amadeus Mozart's Pianoforte Sonata in A minor movement.
As the film opens, the narration on screen tells viewers that the action begins at 3:00 PM. However, when Mortimer & Elaine go up to the window at the marriage bureau, the clerk says "Good morning, children".
Just before Jonathan and Dr. Einstein show up, Aunt Abby arranges the two candelabras so that they are both facing forward. After Jonathan and Dr. Einstein enter, one of the candelabras is facing sideways.
When Mortimer Brewster pushes the policeman O'Hara out the front door, he firmly shuts the door behind the policeman. However, he immediately goes to leave the house and, when he approaches the door, it is wide open.
Dr. Einstein stumbles and falls into the window seat in the dark. He strikes a match, and the wire that is powering the flickering light in the palm of his hand is clearly visible trailing out from his sleeve.
The movie opens with a baseball game, on Halloween, weeks after the end of season. This is a dramatization of a folk truism of the times: "The only way the Brooklyn Dodgers could win is to play on Halloween!" Presumably 1940s audiences would have recognized the joke immediately.