Continuity mistake: At the Scenic Tunnel Marilyn's lover writes a message to her on a postcard and places it back into the rack. She walks over and reads it but it isn't in the same place that he put it as the postcard next to it is different. (00:33:55)
Continuity mistake: When George Loomis comes back to the cabins, after the lover tried to kill him at the falls, he wears the lovers shoes. But after he walks on the grass, just before the end of the shot, the top of the shoes is not white anymore as if he was wearing his own shoes. (00:45:00)
Continuity mistake: Inspector Starkey gets into the police car before Ray Cutler, who sits in the rear driver side seat as they pull away. Several shots later we see them again in a close-up chasing George Loomis and their positions in the rear of the police car have reversed. (01:20:50)
Continuity mistake: When the Canadian motorcycle police drive up and speak to the boat keeper about George Loomis they're all in direct sunlight with no trees anywhere near them. When they leave several shots later it must be an entirely different time of day because shade from the trees across the street covers the entire set. (01:12:25)
Continuity mistake: Two Canadian motorcycle police drive up and speak to the boat keeper about George Loomis, and a prominent tire mark is on the side of the road in front of the black car. The next time we see this spot as Ray and Mr. Kettering run to the boat it's not there. (01:12:20 - 01:17:10)
Continuity mistake: Mr. Qua and the Cutlers walk away from the Loomis' room after talking to Marilyn. If you look closely the stonewall to the left of them is not in direct sunlight as it was a few shots earlier when they walked up to the door. (00:09:45)
Continuity mistake: George Loomis goes over the falls and Polly is safely on an island in the middle of the river with green trees on it. Several shots later the trees are purple. (01:26:35)
Continuity mistake: Before George Loomis strangles Marilyn in the bell tower she knocks everything out of her pocketbook looking for a dime to make a call on the second floor pay phone. We see the items on the floor as George walks past on his way to kill her, but later when he returns and picks up her lipstick everything has been moved. (01:07:25)
Continuity mistake: When the Canadian motorcycle police drive up and speak to the boat keeper about George Loomis, one is wearing a white shirt and the other a blue shirt. In the next close-up shot they're both wearing blue shirts. (01:12:25)
Continuity mistake: The Cutlers drive into the Rainbow Cabins and the manager walks up to them pointing at his clipboard with his right index finger. In the next shot he's holding the clipboard with both hands. (00:06:40)
Continuity mistake: George Loomis tries to steal the boat and pulls the ignition wires straight out from under the lock. No one touches them but the next time we see them they're hanging differently. (01:14:55 - 01:16:10)
Continuity mistake: Ray Cutler drives off to park the car as Polly stands alone in front of the Rainbow Cabin's office. On the left of the screen we see a curled up garden hose leaning against a small pine tree. No one went near it, but earlier it was in front of the pine tree. (00:10:00)
Factual error: Joseph Cotton spends the second half of the movie trying to get out of Canada, but in the scene at The Cave of the Winds with Jean Peters, he must have forgotten that he was already on the American side of the falls. (00:54:00)
Continuity mistake: Ray Cutler decides to leave the Rainbow Cabins and says to Polly, "Give me a hand with this stuff, will ya, Honey". As he says this he picks up a suitcase which has nothing on top of it, but when we saw it earlier in the scene there were clothes on top of it. (00:49:35)
Continuity mistake: George Loomis throws a lit cigarette at the ash tray on the table to the right of the door as Polly Cutler knocks. We see the cigarette smoldering on the table but two shots later after she enters we see the same spot again and it's gone without anyone going near it. (00:20:50)
Visible crew/equipment: George Loomis talks to Polly in the boat and looking outside the window it's easy to see that there's no island in front of them. A few shots later it's obvious that the island is superimposed on the falls. (01:21:15)
Continuity mistake: During the outdoor party at the Rainbow Cabins Marilyn walks out and asks the DJ to play "Kiss". No one goes near them but the three chairs that she passes on her right move about a foot closer to the record player when she walks away. (00:17:30)
Continuity mistake: Polly is safely on an island in the middle of the river as we see the helicopter coming to rescue her. Unfortunately when we see her they're showing us the falls from the wrong direction and we can see the island she's supposed to be on behind her. (01:26:35)
Continuity mistake: As George Loomis tries to steal the boat there's an ice bucket underneath the table to the right of the steering wheel. Later Polly comes back with some beer and grabs for the ice bucket, which has been rotated 45 degrees even though no one went near it. (01:15:35)
Continuity mistake: George Loomis strangles Marilyn who falls to the floor of in the bell tower holding a yellow scarf near her face. In the next shot it's held away from her body. (01:06:25)
