Among the campaign ribbons on Virgil's uniform is an unlikely one for a sailor. The red white and black one is the Berlin Occupation Medal which is unlikely to be awarded to a sailor without some extraordinary circumstances. The closest Berlin comes to the ocean is Bremerhaven 180 miles away.
There are dozens of Christmas presents under Mark's tree and he brings home another dozen or so at the start of the movie, yet over the entire Christmas holiday, only two people are given a present, and there are only three other people--his gopher Virgil, housekeeper Georgette and his lawyer Harvey Butterworth--who might conceivably receive one.
Susan kissed Mr. Christopher under holly, not mistletoe. Mistletoe has white berries.
When Virgil moves from the bedroom to living room, holding a tea cup, there is no spoon protruding from the cup, then, in transiting the living room, there is. He answers the door and there is no spoon again.
When the maid, Georgette, Maidie Norman, reads from the newspaper, there's no printing on the side that faces the camera.
When Isabella confronts new bride Susan in her bedroom, a part of the camera setup and a crew member's arm are visible in the mirror.
When Sgt. Hanlon and Sgt. Maizel barge into Mark's apartment, the camera follows and its moving shadow is visible on the right of the doorway.
When Susan is tussling with Sgt. Hanlon in Mark's apartment, the camera spins around them and its moving shadow is visible on the back of Sgt. Hanlon's overcoat.