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: Ray Cutler decides they should leave the Rainbow Cabins and picks up one of two stacks of books on a table to the right of the front door as he talks to Polly. No one goes near this spot but earlier in the scene there was only one stack of books along with a pocketbook (which is now on the floor) on the table. (00:49:45)
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: George Loomis chases Polly at the Cave Of The Winds and she almost gets killed braking apart the wooden rail as she runs into it. The part of the railing hanging on the left changes position between shots. (00:54:40)
Continuity mistake: Marilyn sleeps in her hospital bed as the song "Kiss" plays from the bell tower. At first she's sleeping on her back then in the next shot she's on her side. (00:57:15)
Continuity mistake: Marilyn walks up to the ticket booth and gets a refund for one of the tickets to Chicago that she bought earlier in the movie. Looking at her luggage we see no marks on it at all. She picks it up the same way she puts it down, but in the next shot a large black mark appears on the front of it. (01:01:15)
Continuity mistake: In the bell tower the shadow is at about 45 degrees, but when Marilyn walks there the sun is overhead. (01:03:10 - 01:05:40)
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)
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: After fishing on the Niagara river with the Cutler's, Mr. Kettering pulls his boat up to a dock that is in the shade. In the next shot the dock is in the sunlight. (01:11:30)
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: 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: 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: As the Canadian motorcycle police drive away George Loomis hides across the street behind a boat and some items that look like they belong in a tag sale. In the next shot as he walks away all of the items on the right of the boat have been rearranged. (01:13:10)
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: 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: The gash on Lomas' head changes between scenes. (01:16:15)
Continuity mistake: The boat keeper throws his newspaper onto the ground as he runs to the boat that George Loomis is stealing. Later when Ray Cutler runs past that spot the newspaper is now on the chair. (01:16:25 - 01:17:15)
Continuity mistake: George Loomis gets the engine of the Myra III running then proceeds to untie the two ropes from the dock leaving them dangling off the side of it. In the next shot as the boat sails away we see no ropes. (01:16:30)
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)
