Deliberate mistake: In Raymond's flashback to "when he was once loved" there is a scene where he is having dinner with the Senator and his daughter. Evidently someone didn't feel that this scene was quite long enough as shot. Raymond picks up a piece of meat on his fork, puts it in his mouth and then begins to laugh. Then he puts the empty fork back into his mouth, removes the piece of meat and replaces it onto his plate. Obviously, the film was reversed briefly to give the illusion of a longer scene.
Deliberate mistake: When Humbert is walking through Quilty's mansion at the start of the movie, a bottle is placed near the top of a chair covered in a sheet, which is later revealed to be covering Quilty; the bottle is also shown to be placed on top of Quilty's head. This same scene is shown a second time at the end of the movie, but the bottle near the top of the chair cannot be seen. There is a second bottle on Quilty's lap that can be seen in both the two scenes, but the bottle on Quilty's head cannot be seen the second time around. The two scenes were likely filmed two seperate times. (00:02:50 - 02:32:18)