Revealing mistake: When Marion is driving her car through the night, she keeps looking at her speedometer. You can clearly see that the gear shift lever is in "park" as she is driving.
Revealing mistake: After Marion has been murdered there is a close-up of her face. If you look down to her neck you can see her pulse.
Revealing mistake: After Marion's sister and boyfriend look through the Number 1 bathroom and find the ring on the toilet, they turn off the light. On the wall behind them you can make out a square beam of light, but no other lights are on in the cabin.
Revealing mistake: When Marion's boyfriend and sister visit the Sheriff and his wife at home, there is a camera shot where you can see that the living room background scenery ends and there is no ceiling.
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