Continuity mistake: Before Tony leaves he closes the drapes. He then says he won't go out and will stay home. She says to go out, and he agrees, so he goes to the drapes and closes them again.
Continuity mistake: When Tony throws the 100 pounds down-payment across the room to Swan, watch the chair in which it lands. It has a low back one in one shot, then a high back (probably to provide a bigger target for the throw), then a low back again.
Visible crew/equipment: While Tony is blackmailing Charles Swann, describing the story in the newspapers regarding the "middle-aged woman found dead due to an overdose...", just as Tony takes a seat on the table we can see the moving shadow of the boom mic on the wall, over the picture. (00:25:10)
Continuity mistake: When Mark is talking to Margo early in the film in her living room, the camera shows a front view of her and she is standing in front of liquor bottles lined up on a table. Behind her, Mark is in front of the fireplace. When Tony comes home within minutes, the bar is next to the front door and the bottles are rearranged.
Other mistake: The judge in giving sentence refers to the murder occurring on the 27th September. Towards the end of the film the implication is that the murder occurred on the 27th March e.g. he started living on the cash from the 27th March.
Continuity mistake: When Wendice returns to make his final attempt to open the door with the latch key hidden under by the stair, reaches for a key that is located at the second ornate wrought-iron scroll step which is where he left it originally. When Swan reaches for the key in the dark hallway, it is now hidden at the third ornate wrought-iron scroll step, which is one scroll higher. They keys are not in the same place. (00:40:00 - 01:44:00)
Factual error: The Inspector has no authority at all to release a convicted murderer about to be executed. Even if he could, the police presence would have been greater and Margot would have been in prison clothes or at least in restraints.
Plot hole: The ending relies on Tony knowing where the key is on the stairs and Margot not knowing. Unfortunately there are any number of reasons for him knowing about a backup key that do not mean he is a murderer, and she could be smart enough to know she is certainly under police surveillance (and custody) and not touch the key on the stairs. Tony could easily make the argument that he had one made as a backup and put it there. Swann could have followed him one day and saw it and used it. His knowing the location of the key certainly does not necessarily mean he hired a hitman.