Revealing mistake: In the scene where Norman visits Caleb at the motel on his own, after hearing Norma and Dylan argue, he gets into an argument with Caleb. Norman then pulls a knife out high above his head, at which point you see it is a prop/rubber type knife. The handle aimed at Caleb but the force Freddie Highmore has used to pull the knife up and back causes the blade to bend and aim toward the ceiling.
Audio problem: After Deputy Lin drops the line "we'll get the results back fast," Sheriff Alex Romero responds with "I'm not sure how much nail clippings are gonna tell us." As he sits down, his mouth movement matches the audio, but when the angle changes, his mouth isn't moving at all. (00:09:55)