In a scene late in the film, where Lauren Bacall and Charles Boyer are driving in the car, one angle shows a two-tone steering wheel (amber on top, ivory on the bottom/grip side); but in the next shot, a closeup on the steering wheel, the entire steering wheel is ivory.
A "silencer" is shown being used on a revolver. Due to how a revolver is constructed silencers won't work on most of them.
Early in the film when Luis meets Licata on the ferry at the bottom of the steps to the first class section, there is a chain and sign across the entrance to the steps. However, Licata walks away from Luis after they talk without undoing the chain or stepping over it - as if the chain wasn't there.
Just after Inspectors try to take Boyer in custody he leaves the house and Charles Boyer is evading Inspectors in the fog, the police car passes him and the stunt rope attached to the back of the car is clearly seen being pulled as it passes. can be viewed in movie starting at min. mark 1:21:00
When Charles Boyer pushes Peter Lorre into a cab and says, "Let's get into this taxi," the reflection of a crew member wearing short sleeves can be seen in the reflection of the cab window, facing the actors.
Bacall and Boyer agree to meet at the crossing of Regent Street and Piccadilly. But no one would simply say this. This intersection is huge, and is always crowded, so one would have to specify the corner. (In the book they met at the Russell Square tube station.)