When David receives the letter from child services, the envelope not only has six digits in the zip code instead of five, the state is listed as "CS", which does not exist. However, an early scene includes a tight shot of the front license plate of David's car, which says at the top "COASTAL STATE" where a real license plate would say "OREGON" or whatever. The production designers invented a state, a postal abbreviation (CS), and a ZIP code; bravo for this attention to detail and imagination!
While David and Dennis are driving David's sunglasses change from red when looking from Dennis' point of view to black when looking from David's. When they hit the police car and the police approach the glasses are black again.
When David and his sister are looking at the projector Dennis created in his room it is clearly has a single light projecting each of four pictures through a magnifying glass as they rotate past the light, but when they look up we see all four photos projected and rotating on the ceiling.