Other mistake: In the shower scene, Marion Crane's palm looks water wrinkled as if she's been in the shower for quite a long time even though she was only in for a few moments before Norman stabs her. Also, when Norman drags her out of the bathroom, Marion's legs are stiff and together and she has her toes pointed.
Other mistake: If the film begins on Friday, December 11th and continues to the following week, there should be more Christmas decorations, not just the ones seen on the store when Mr. Lowry walks by Marion's car.
Answer: Norman and his mother lived together in the house on the hill above the motel. Norman's mother was such a demanding, controlling, overbearing woman that Norman was eventually driven to kill her. The enourmous guilt of this action, combined with the strain he was already under from her treatment of him, caused him to develop a sort of modified split personality: in addition to the existing Norman, he constantly heard his mother's voice in his mind and even had conversations with it. As time passed, the "Mother" voice in his brain began to have some occasional control over his body. Thus, sometimes Norman was in control, sometimes his mother was in control, and sometimes they could "speak" back and forth within his mind. Norman checked Marion into the hotel, but the sexual attraction caused by her presence made his disapproving Mother personality manifest and kill Marion. Having dealt with the situation, Mother retreated and Norman cleaned up, not remembering. At the end of the film, Mother blames Norman for the crimes because she is once again controlling his life and not wanting to take the blame herself. At the same time, this represents Mother forcing Norman down to some corner of his consciousness or even destroying it altogether, meaning that it is unlikely that Norman will ever manifest control again. This is why we hear Mother's voiceover and not Norman's at the end.
Phoenix