When in the railroad station Asta runs after the other dog and then sits down; her leash is curled up in front of her. However, on the next cut, the leash is now straightened out behind her.
When Mary is holding her rifle up over her shoulder as if about to hit Nick Charles and then reacts to the mention of the name Eleanor Morgan; she lowers it. However, in the next cut after Nick acknowledges knowing about Eleanor; the rifle is still raised over her shoulder, and then she repeats the action of lowering it.
When Nick places Mrs. Draque on the bed; she has her legs off to the side of the bed. However, on the next cut, her legs are flat on the bed.
In the baggage car scene, Asta twice hides behind a milk can - once running from the right and the second time, running from the left. In the second sequence, the writing on the crates behind him is backwards. This is because the film editor simply reversed the negative from the first sequence in order to "create" the second. [Note: though, in the novel, Asta was a female schnauzer, in the movies, the dog is a male wire fox terrier.]
When Nick and Nora are in the baggage car, the train passes a windmill. The scene outside is being shown on a rear-screen projection, but the camera shooting it oddly and obviously pans when passing said windmill.
The cake that Nick's mother carries in for Nick's birthday is clearly a prop. The actress holds it not from its bottom, but from its sides - if the cake were real, the tray on which it sits would fall to the floor. Then, in one continuous take, she sets it onto the table and embraces Nick, but her hands are clean of the "cake's" frosting.
It has been said that the rifle in the film was a Bren gun and not a Japanese weapon. However, the Nambu machine gun looks very much like a Bren gun. The stock is the most easily spotted give-away. The Bren gun's stock come straight back from behind the pistol grip right behind the trigger guard, while the Nambu's stock has a slight drop to it, right behind the pistol grip. The gun in this film is, in fact, a Japanese Nambu machine gun.
After Peter Berton is shot and killed, Nick Charles goes to search his room at Tom's Auto Court. Tom tells him that there are no vacancies. Nick explains that there is one now because Peter Berton was shot. Before he searches the room, he switches room numbers to fool the police chief. The police chief is fooled and searches the wrong room, complaining to Nick later what a dirty trick that was. If Peter Berton's room was the only vacancy, the room that the police chief searched would have been occupied.
In the gift shop scene, Willie Crump, played by Donald Meek, has an obvious rip in the sleeve hem of his jacket, on the back right shoulder. His white shirt shows through. It can be seen as he gets up from having his head inside the display case.
When Nick is repairing the table he does a couple of counter clockwise turns and then one slow, firm counterclockwise turn, seemingly making the screw very tight. This was intentionally tongue in cheek, as one must turn a screw clockwise to tighten it..
When Brophy is sitting with Nick's parents at the table, he is describing a card announcing births. During his description, he takes a bite of cake, but the dialog continues non stop, even though his mouth is obviously not mouthing the words.
When Peter Berton is standing in the doorway talking to Nick, when he speaks, the words come out, "You see, there's something I've gotta get off my mind." But the mouth movements indicate he said, "There's something I've gotta get off my chest."
At the very end of the film Hilda the maid asks Nick for his autograph. She hands him a pad to sign and it looks like the film rewinds or it was not edited properly and appears she pushes it toward him and he pushes it back to her.
At the end of the film as Nicky's vest starts popping buttons; you hear a popping sound effect. However, four buttons come off but only the first and third button popping sound is heard. There is no sound effect for the second and fourth button popping indicating an error by the Foley artist.