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: 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)
Visible crew/equipment: The scene when Jean Peters jumps onto the little island is very well lit. You can see the reflections of 10 or 15 spotlights on her. (01:25:40)
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: 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)
