Plot hole: Patrick Redfern needs someone to be in the boat with him to corroborate his and his wife's deception and be his alibi. That is whole point of his wife lying on beach - he gets this when taking Myra for a ride - but it was portrayed as a random ride she got, he did not intend or plan giving her a ride, he did not see her loitering near the boat, or even in view of him untying the boat. Even if that was intention in the story for him to be seen ready to cast off, the movie viewers did not see her viewing him, only her appearing just before he was set to cast off. Without someone with him the whole deception would not work, he would have no alibi, he could have killed Arlena unseen.
Continuity mistake: In one scene, Nick Nolte is driving Eddie Murphy around in a convertible. First you see Murphy in handcuffs, then no cuffs, he's got one arm on the back of the seat, and then back with the handcuffs on.
Continuity mistake: When De Niro's character gets in the limousine with Lewis' character he has a hurt hand. He wraps it with a handkerchief. As the scene progresses the handkerchief disappears and reappears depending on the camera angle. (00:07:15)
Continuity mistake: When Terry Corrigan goes mad and teaches his students at gun point, he asks one of the thugs a question and holds his gun right up to his face. The tip of the gun is almost touching the thug's face, but from a different angle, the gun is further away.
Continuity mistake: Closeau is at his desk preparing to light his pipe. The Pipe is in his mouth as he lights the match, the camera angle changes and now it is out of his mouth in his left hand as the camera zooms in, he puts it back in his mouth to light.
Factual error: Kathleen Quinlan is named Janet Dunn, but when Gene Wilder looks at the newspaper and sees the headline about her murder, it falsely says her name is Janet Stone.
Continuity mistake: Myra Bruhl is popping pills at the beginning of the movie; when she puts down the container, there's a white-labeled bottle on the bedstand that was not there before. (00:02:40)
Revealing mistake: Throughout the movie, practically every victim ends up with a broken, twisted neck. The "special effect" used for those deaths is...the actors wear their jackets backwards. It is made even more evident by the plot twist at the end.
Continuity mistake: John is helping Mr. Nardini at the beginning of the movie. The box breaks right away and spills its content on the doorsteps, and the various items (pay attention to the blue bags of pasta for instance) shift position at every cut. (00:03:50)
Continuity mistake: Near the beginning when the guy is robbing the liquor store, he's holding his gun with one hand. In the next shot, he's holding it with two hands.
Visible crew/equipment: When the plane drops the grenade into Halicki's car, and he throws it through the sunroof, it blows up the car tailing him. In the slow motion shot following, the stuntman's crash helmet is visible. (00:50:00)
Character mistake: During the surgery, one of the surgeons says to give the patient 25 cc's of cyatril. Then one says make it 35 cc's. 35 cc's (or 35 mL) is a full large syringe. The other doctor uses what looks like a 3 mL syringe and it's only partly filled. It seems the surgeons wanted 0.35 cc's.