When Jo climbs down into Mappin & Webb, he is the fourth and final member of the gang. Yet you can see another pair of legs still in the room above.
When Tony and Mario go out in the black Citroen to time the drive with a stopwatch, they start out in a car with the standard grille and plate # 2126 DB75, but after the first turn they are in a car with an after-market grille and plate # 3510 BU75.
Tony's pistol switches from a Walther P38 to a Luger P08 in alternate shots.
When they get in the car to drive to the heist, one of the guy's coats partially hangs out of the car when the door is closed. However, when they're driving there is no coat to be seen.
When the "modest" Mario gets out of the bathtub, one can see briefly that he has boxer shorts on.
When Tony picks some dirt out of the dead Jo's hair, Jo's right eye twitches.
When the injured Tony is driving through the streets, in several shots, every person on the sidewalk in the background is looking at the car and the camera vehicle doing the filming.
When Pierre enters Mado's dressing room, the boom mike's shadow moves across the top frame of her mirror.