When Sondra sits in the boat with Peter before he pushes her into the lake she wears beige / light brown clothes. When she returns to the house and happens upon Peter and the policemen, being still wet, her clothes are dark brown, almost black. That's too dark to be wet beige. It can't be muddy either, because the rest of her (hands, hair, face) are clean.
The instrument in the Music Room under which the Tarot cards are hidden is constantly referred to as a "French horn" when in fact it is actually a Conn 16E Mellophonium. Additionally, in the first scene in which the Mellophonium is shown it appears without a mouthpiece. In all subsequent scenes a mouthpiece is in place.
Sondra tells Peter Lyman that her father is having sensitivity in tooth "lower # 7." In the American tooth numbering system, number seven is always an upper tooth, specifically the upper right lateral incisor. In the British tooth numbering system, there is no tooth number seven.