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.
While Mortimer talks to his aunts early on, the rice in his hair disappears and moves around between shots.
A policeman named Rooney has his rank fluctuate between "Lieutenant" and "Captain" throughout the film.
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.
When Mortimer discovers the body in the window seat his hair changes from being flipped over his forehead to being neat and then back again without any obvious motions that would have changed it.
(at around 6 mins) When Teddy is playing the harpsichord, the music on the stand is a duet for an instrumentalist with a keyboard accompaniment; hence the three music staves, yet Wolfgang Amadeus Mozart's "Piano Sonata No. 8 in A minor, K. 310 (1st Movement)" is heard.
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.
When Jonathan runs his thumb along the edge of the surgical knife, it does not actually touch the blade.
(at around 1h 50 mins) When Mr. Witherspoon (Edward Everett Horton) picks up the bugle, he also makes a point of picking up a glass that had fallen over and putting it upright on the table. Presumably this detail wasn't in the script, as Mortimer (Cary Grant) can be heard muttering, "Eh? He wasn't supposed to do that." or something very similar.
When Jonathan describes Dr. Einstein as "somewhat of a magician," his lips aren't moving.
When Mortimer opens (for the first time) the window seat, then closes it, he walks away and his voice is heard saying "The next thing..." before trailing off, but his lips do not move.
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 visible trailing out from his sleeve.
When Mortimer runs into and flips over the chair, it can be seen that large hinges are attached from two legs of the chair to the floor, stopping it from flipping completely.
As the film opens, the narration on screen tells viewers that the action begins at 3:00 p.m.. However, when Mortimer and Elaine go up to the window at the marriage bureau, the clerk says, "Good morning, children."