Anachronisms: One of the negatives that Mr. Arbus is developing is a Kodak T-MAX 400, which was not available in 1958.
Revealing mistakes: Towards the end of the movie, Lionel is shown beginning to blow up the canvas raft. He later explains that it is for Diane when he takes his final swim. Someone suffering from such extremely low lung function that he will only live a few months would never be able to inflate a raft that size.