When Ellen opens the bookstore, there is a blue USPS mailbox right in front of the entrance. Yet, there is none when the establishing shot is filming the front of Buy The Book.
When Harold is sitting at the diner at the end, the waitress sits down to talk to him. When she gets up, there are two upright teacups and saucers, but he is sitting alone. Obviously, this was premature: the tea cup on Ellie's table is upside down on its saucer, so for her to sit at her father's table in front of the second cup, when the waitress had been talking over it, is unsanitary. (The waitress later pours coffee in it for Ellen.)
When Ellen is telling her mother to signal her, she replies but---although Lois is mouthing words---no audio is heard but Ellen's voice is.