When Phillip and Brandon put David in the chest, the rope is clearly around David's neck and completely inside the box. But in a few minutes Phillip finds the rope hanging, very far, outside the box.
Philip cuts his hand on glass, but moments later there's no blood or wound visible.
During the party, the chest that houses David is tall enough to serve food from. However, by the end, as Rupert crosses to the chair next to it, the chest is only as tall as Rupert's knees.
When Rupert is talking to Brandon and holding two plates of ice cream, the camera moves behind Brandon's back to make a cut. When the camera re-emerges, Rupert and Brandon are standing in the same positions, but the doorway background is in a different place. Also, the ice cream topping has changed from chocolate to caramel. Furthermore, the piece of cake changes position on the plate in Rupert's left hand - it moves to the opposite side.
With a cigarette in his left hand, Brandon makes a quick phone call to the garage man. After he hangs up the phone he has no cigarette.
When the film opens, the view is of the window of the apartment from the outside. The scream of a man is heard and then the scene changes to that of Brandon and Philip strangling David with a rope. But if one is being strangled one can not scream. The rope would have crushed down on the victim's trachea and cut off the flow of air (in either direction).
The newly dead body would have voided itself and thus there would have been a distinctly noticeable smell. Also, it is also not unusual for a newly deceased body to intermittently makes sounds as gases are created and move about.
During the opening credits - and judging by the windows on the opposite side of the street - the camera appears to be positioned roughly at the third, possibly fourth floor height at the most. However, the window view throughout the film suggests a flat on a much higher floor.
As Brandon and Philip walk back to the kitchen after finding the rope, the frame over the doorway into the hall can be seen separating. Since each reel was filmed in one continuous take, sets had to be moved out of the way during the shot, allowing the camera to follow the actors between rooms. (A similar mistake is visible in the tracking shot out of Bob Rusk's house in Frenzy (1972).)
When Brandon is opening the curtains of the long window after killing David, the shadows are in the wrong direction, towards the window.
Joan Chandler looks directly at the camera a handful of times.
Before Brandon opens the curtains on the long windows before the party no traffic noise can be heard. After opening the curtains, and without opening the windows, street noise can clearly be heard.
Philip's piano playing does not match the music playing.
A camera shadow is visible on Rupert as he gives his speech at the chest near the end of the film.
When Brandon is opening the curtains of the long window after killing David, the shadow of an arm can be glimpsed behind the curtain in the upper left corner of the window.
In a few scenes, if you look closely at the carpet there are white markers dotted about.