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: The Rainbow Cabins are on the Canadian side of the falls and when Ray Cutler looks over at the shredded wheat plant he should be looking at the American side. But the error here is that they show the Sir Adam Beck Generating Station which is also on the Canadian side of the falls. (00:06:50)
Continuity mistake: After they leave the customs officer Ray Cutler says to Polly, with a wide grin on his face, "You should have told him we're on a delayed honeymoon". In the next shot he's not smiling at all. (00:06:05)
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: 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: Both Marilyn and Jean's faces are dry as bones in the scene below Niagara Falls but Marilyn's lover is drenched. (00:13:50)
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: 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: As Polly and George Loomis are rushing down the middle of the Niagara River a tree appears above Polly's head on the back of the boat. (01:23:10)
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)
Continuity mistake: At night the Rainbow Cabins has a little outdoor party. The Cutlers are dancing together, and when the song stops Ray has his arms around Polly's shoulders. In the next shot his arms are around her waist. (00:16:35)
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: At the bottom of the Scenic Tunnel Tour, Polly runs into Marilyn kissing her lover. At first the gentleman has his hands around her waist. Then in the next shot his right hand is holding her face. (00:13:45)
Continuity mistake: In the first scene of the movie Marilyn is in bed smoking a cigarette and the shadow of the light coming through the Venetian blinds goes across the sheets from left to right. She hears George coming into the room, puts out the cigarette and in the next shot the shadow from the blinds has been rotated 90 degrees and is now extending from the foot of the bed to the head board. (00:03:20)
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 Cutler enters cabin B in order to put some Mercurochrome on George Loomis' cut and the do not disturb sign on the door knob is tilted to the left. No one goes near it but later in the scene it's been set straight. (00:22:55)
Continuity mistake: The manager knocks on the door to cabin B and Marilyn comes walking out. The shades to her right are closed but two shots later they're opened. (00:08:25)
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: 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: The gash on Lomas' head changes between scenes. (01:16:15)
