Trivia: Not a mistake, but another elf sighting within the movie. In the Parent's Day scene in Charlie's classroom, watch the boy sitting directly behind Charlie in the blue shirt. When Charlie walks back to his desk to retrieve his glass ball to show everyone, you can see the boy's pointed ears. Another note of interest is when Charlie says "my Dad is Santa Claus" and it cuts to a wide shot showing all of the kids, this kid is the only one not laughing.
Trivia: Not a mistake, just a little fact. Throughout the whole movie, the elves from the North Pole can be spotted in different scenes. For example, when Scott is talking to Charlie about secrets while walking down a sidewalk, there is an elf girl behind them walking in a different direction who looks at Scott, then turns around. Another incident is when the same elf girl is standing behind the bench at the soccer game when the children are lined up to sit in Scott's lap.