Continuity mistake: After George Loomis shuts the lights Polly Cutler looks to the left at the falls in the dark, through the back window. This is the wrong direction as we see later in the movie as Marilyn has to look to the right to see the falls during the day. (00:23:15 - 00:36:15)
Continuity mistake: George Loomis goes over to the front door and shuts the light so that Polly Cutler can see the falls in the dark. The blinds on the left of the door are half open but when we see them again later in the scene, they're fully open even though no one has gone near them. (00:24:05)
Continuity mistake: In bed in their room Marilyn says, "Get me some orange juice Georgie" as she lays in bed on her side. In the next shot she's lying on her back. (00:29:30)
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: Marilyn's lover writes a one line message to her on a postcard and places it back into the postcard rack. She walks over and reads it and we now see that there are 6 lines written on the back of the card. (00:34:05)
Continuity mistake: Ray Cutler runs out of the cabin office and says to Polly, "How do you like that?". In the overhead shot you can see tire marks on the gravel driveway but in the next shot there are none. (00:34:30)
Continuity mistake: At the Rainbow Cabins, Polly Cutler lays out on a beach chair sunning herself as Ray comes out to talk to her. There's a second chair about 10 feet to her right but the in the next shot it moves right up next to her. (00:34:40)
Continuity mistake: As Polly Cutler lays out on a beach chair sunning herself Ray steps back and decides to take a picture of her sitting there. Niagara Falls, which are always seen behind them are not in this shot, which has obviously been filmed on a sound stage. The bushes against the stone pillar are also different. (00:34:55)
Continuity mistake: Several dark marks appear then disappear and then reappear at different times on the Loomis's front door to cabin B. The marks are there during the party when George Loomis storms out and breaks the record, but then they're not there as Marilyn walks back into the room after speaking to the Cutler's. (00:36:05)
Factual error: Marilyn and the Cutler's follow Inspector Starkey down to the bottom of The Scenic Tunnel Trip to identify George's shoes and when they come up Marilyn hears the song "Kiss" chiming from the bell tower. This is an obvious mistake as the shot of the bell tower that we see here is taken from the American side of the falls and The Scenic Tunnel Trip is on the Canadian side of the falls. (00:39:55)
Continuity mistake: As Marilyn walks into room 7-A of the City Morgue to identify her husband George she's carrying her pocketbook in her right hand. After she sees the body she faints and falls to the floor but we never hear the pocketbook hit the floor because it has been neatly placed next to her before she fell. (00:42:05)
Continuity mistake: At the Rainbow Cabins, Mrs. Kettering hands Polly Cutler a bouquet of roses and the pink bow is on the left. In the next shot the bow is rotated 45 degrees to the right and is now facing Mrs. Kettering. (00:44:10)
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: At the Rainbow Cabins, Polly Cutler speaks to the Kettering's and holds a bouquet throughout the scene, and it keeps changing positions between shots. (00:45:05)
Continuity mistake: Mr. Qua is watering the flowers and as Polly Cutler speaks to the Kettering's we see the front of the car and there's no water on the gravel by the tire. After they get into the car and drive away in the next shot, there's a lot of water on the gravel. (00:45:15)
Other mistake: The Cutler's now occupy cabin B at the Rainbow Cabins and just before we see George Loomis by the garbage cans in the back of the cabin Polly decides to take a nap. When she gets up from the bed to close the Venetian blinds we see the pillow case and it's white, but two shots later after the blinds are closed she lays down again and the pillow case has a huge dark make up stain on it from several previous takes. (00:46:20)
Visible crew/equipment: After his supposed death we see George Loomis back at the Rainbow Cabins and he's standing by the garbage cans in the back of cabin B. We see him through a window and if you look closely a crew member's thumb can be seen on the window at the beginning of the shot. (00:46:35)
Continuity mistake: Polly decides to take a nap in cabin B and the ash tray on the night table moves to the edge during the scene without her going near it. (00:46:45)
Other mistake: The first time we see George Loomis back at the Rainbow Cabins after his supposed death he's by the garbage cans in the back of cabin B with a brick wall on the left. The next time we see him two shots later the film has been reversed with the brick wall now being on the right. (00:46:45)
Continuity mistake: Sunlight is coming into this room from all directions. At first it is coming from the right of the screen. Then when Denis O'Dea goes into the kitchen (screen left), sunlight is coming from the other direction. (00:48:40)
