Revealing mistake: After Nick escapes from the agents, he runs to a convenience store and bangs on the door for help. While he's doing this, his reflection can be seen on the glass of the door he's trying to open.
Revealing mistake: When Nick is running from the men in the park and has his hood off, he is running away and appears to have no head. His torso is visibly longer than a normal person's. The special effect used to make this scene involved a man with an elongated torso to hide his head.
Revealing mistake: When Nick returns to his apartment (after being made invisible) he immediately closes the curtains. Although he is invisible, we should see the curtains pulled closed at the point that he grabs them with his hands. Instead we see them close from the top, revealing they were closed either by a crewman or an automated process. Note: The curtains are NOT meant to be automatic in the film as we see and hear him physically close them.
Audio problem: In a scene where Chevy Chase is smoking a cigarette, we see smoke going in and out of his lungs. He speaks at the same time that he inhales the smoke.