(at around 1h 45 mins) There is a rear shot of Paul standing outside Betsy's cottage door, arms folded behind him. As Paul calls out to Betsy, he unclasps his hands and brings them forward. Then from the frontal view of Paul through Betsy's screen door, his arms are folded behind him, his hands out of sight.
During the opening sequence, when George and Ann are listening to CDs in the car the shadows of the car and boat indicate that the sun is alternating between side-on/quite low in the sky to directly behind the car and almost overhead.
When an is talking to her friend on the phone she mentions that the kitchen clock is broken and it needs fixed. The clock is clearly visible on the shelf to her right. When Peter/Tom comes back into the kitchen for eggs, the clock is missing.
Throughout the movie Paul calls Peter "Tom", and Peter calls Paul "Jerry". These are references to the Tom & Jerry cartoons. They also refer to each other as 'Beavis' and 'Butthead', another cartoon reference.
The goof item below may give away important plot points.
The mobile phone the couple attempt to use to call the Police after their son's shooting is a Nokia. When switched on, a Nokia start up tone is used. However, the key tone is that of a Motorola (the tone is heard when they press the Call button after 911).