Continuity mistake: When Marion is buying another car at the dealership, in the background you can see the same extras walking in different directions on the sidewalk.
Continuity mistake: When Norman carries Marion's dead body out to the boot of the car, the water on Marion's bonnet changes between shots. In some shots there is water, sometimes there is no water.
Continuity mistake: When Marion steps into the shower and pulls the curtain shut, if you look closely, you can see a rubber bath mat with holes in it. After this shot, the mat is not seen again (during the murder or the clean-up). (00:46:20)
Audio problem: When Norman runs down from the Bates house to the motel and meets Marion, he says "Gee, I'm sorry I didn't hear you in all this rain. Go ahead in, please." But you look closely, it looks like Norman is barely moving his lips. In fact it looks like Marion is saying something instead. (00:27:50)
Continuity mistake: While in the shower, Marion rubs her right arm with a bar of soap and her hair has yet to get wet. But in the next shot, the soap has disappeared from her hand and most of her hair is wet. (00:46:45)
Continuity mistake: When Marion gets out of her car at the dealership, she is holding her purse with her hands. But in the next shot, her purse is now around her left arm. (00:17:40)
Plot hole: When the cop comes back after making the phone call, he's just missed Norman. He heads over to the office and calls out for Bates. He's in there around 45 seconds. Then he leaves and heads towards the big house. How can Norman be at the house ready to kill him in his mothers clothes, when he had no idea the cop was back and he would not have had time to get there from where he was, right at the far end of the cabins?
Continuity mistake: When the Highway Patrol Officer hands the driver license back to Marion, in the interior shot he's holding it out with his right hand and Marion takes it with her left hand, but in the next exterior shot he's handing it to her with his left hand and Marion is taking it with her right hand. (00:15:50)
Continuity mistake: In the scene after Norman kills Marion, when he goes back into cabin 1 to move her body and clean the bathroom, look on the floor between the bathtub and the toilet and you'll see a pair of bathroom slippers. As he mops the bathtub and then dries off the floor with a towel the slippers magically disappear. (00:52:55 - 00:54:05)
Continuity mistake: The shadows made by the sunlight streaming onto the hotel office wall completely change, from the scene in which Sam grills Norman about the $40K, to the shot in which Norman runs out to find Lila. While Sam and Norman were in the hotel parlor and Lila was in the house, who pulled the office window shade half way down - Mother?