Deliberate mistake: When Norman picks up Marion's supposedly naked body to carry it to his trunk, a bra is visible through the shower curtain. Gus Van Sant included this goof in his 1998 remake.
Deliberate mistake: Mrs. Bates is in a rocking chair, but in the close-up that reveals her face her position smoothly rotates as though she were in a swivel chair.
Deliberate mistake: After being stabbed, Arbogast falls down the stairs in an impossible manner. He is standing tilted backwards all the way down the stairs.
Chosen answer: Birds are just a favourite choice of spooky animal that Alfred Hitchcock seems to use hence the film "The Birds". Eyes show the window to our soul where our deepest fears originate and who isn't scared of getting knifed in the shower. The mirrors i can't explain but I can suggest that it just looks creepy.