When Barnaby is in the hospital room with Amy the close up of him is printed backwards, as his hair is suddenly combed in the wrong direction. This may have been done deliberately because if printed correctly, it would have looked like he was facing away from her while talking, and there presumably wasn't time to filmed him again later.
When Barnaby tells Amy that he found her apartment keys he holds them out to give to her. Belinda Montgomery momentarily forgets she's playing a blind person, as she instinctively reaches directly for the keys and doesn't waive her hand around like a blind person searching for them.
Toward the end at the Cleveland house when Barnaby decides to go outside to confront the person in the bushes, he instinctively heads for a door on a side wall. Barnaby had never been to this house before and wouldn't have automatically known where the door was, especially since the wall was all wood paneling and the presence of a door was not obvious.
When talking to Barnaby Ellen makes a common mistakes of using the word "inferring" instead of the correct "implying".