Factual error: Right after the opening credits, captions say "Susan's house - 1961." And right afterwards, the model train runs by two cigar boxes, one of Dunhill n.25 (seemingly one of the later designs of the box) and the other of "Joya de Nicaragua", a brand founded in 1968. (00:01:25)
Audio problem: When Susan's cutting the cake in the kitchen she drops the plates upon discovering David's body yet there's no sound of them breaking. (00:04:25)
Suggested correction: Actually, I just watched the scene and the sound effect of plates dropping/breaking is present and audible (even through the annoying Pyscho-like strings riff).
Continuity mistake: When we see Susan and her friend David laughing, Harold's at the closed window. When Susan comes back into the room the window's open.
Suggested correction: I mean...yes, but he also got inside the house and murdered the kid in the meantime (making loud noise in two separate instances). The implication is that he opened that window to get in, no?