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 Mr. Witherspoon is getting up from the table, he stands an item up on the table which had been knocked over. Mr. Witherspoon then gazes up with a look as if to say, "Oops, I wasn't supposed to do that. Are we going on with this scene?" Cary Grant was feeling pretty certain that the scene would need re-shoots as you can distinctly, yet quietly, hear him ask off-screen, "Was he supposed to do that?"
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 hymnal that the aunts use to hold services over Mr. Hoskins is "Hymns For Creative Living," published in 1935 by The Judson Press. The hymn that they sing ("There is a Happy Place") is not in that hymnal.
The movie opens with the Brooklyn Dodgers winning a baseball game on Halloween, weeks after the end of baseball season. This is a gag to suggest that the only time the Brooklyn Dodgers could win is on Halloween, similar to saying when pigs fly.