Anachronisms: The movie is specifically identified as taking place on Hallowe'en (October 31), but opens with a Brooklyn Dodgers baseball scene. Even if the visiting "New York" team is the Yankees, making it a World Series game, it would have to be earlier in the year than that; no World Series from 1913 through the 1940s ran later than October 16, and in fact it was not until 2001 that the Series ran as late as October 31.
Crew or equipment visible: 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.
Continuity: While Mortimer talks to his aunts early in the movie, the rice in his hair disappears and moves around between shots.
Continuity: 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.
Continuity: 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.
Continuity: When Mortimer first thinks of calling Judge Cullman, he raises his left arm. In the next shot, his right arm is raised.
Audio/visual unsynchronized: When Jonathan is trying to find Mr. Spenalzo, he mentions to Elaine that his friend Dr. Einstein is "somewhat of a magician." However when he says this, his lips aren't moving.