Character mistake: When Marion is calculating how much she spent her math is wrong. The movie shows 400000 - 4036 = 395963. It should be 395964.
Continuity mistake: When Sam knocks down Norman wearing a wig, he crouches in pain yet in the following shot he is standing tall.
Other mistake: When Marion Crane trades cars she drives off the used car lot with California license plates on her newly purchased vehicle. The dealer should have transferred her old Arizona license plates to the new car.
Factual error: At the end of the shower murder, the camera pans back from Marion's eye, one of the most iconic scenes in cinema. If we are to presume she is now dead however, the nature of muscles surrounding her pupil would cause them to relax and her pupil would dilate, not go to a pinpoint as depicted.
Other mistake: Norman walks out of his mother's house with an umbrella in his hand in the pouring rain and yet is bone dry when he reaches the motel. The woman as well, despite going back and forth from her car in the pouring rain, remains dry.
Plot hole: Marion rips the paper into pieces and flushes them down the toilet. We see all the paper go into the toilet. Later when Lila lifts the toilet seat, there's one piece conveniently placed under the seat that couldn't have gotten there.
Answer: What is shown is what you would see in the rearview mirror. Being in the mirror, it's a reverse image. Think about if you were looking at the cars pulled over head on/facing the driver, the road and the driver would be on your right. Shots in the mirror though shows the road and the driver on your left, the reverse.
Bishop73