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.
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.
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.
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.
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.