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.
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.
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 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)