Revealing mistake: In the scene where young Norman kills the horny young girl, as he's talking to Mother, she can be seen moving and even blinking her eyes when the camera's at her back, yet at this point in the movie, she's been murdered and placed back in her bed. Clearly the actress missed the memo.
Continuity mistake: Norman pours poison into the pitcher of iced tea. In the next shot, we see him carrying the pitcher and two full glasses on a tray - but the level of liquid in the pitcher hasn't changed, even though he's supposedly filled the glasses from it. Later, when the pitcher is sitting on Mother's dresser, the pitcher's level is lower, but no one has poured anything from it.