Corrected entry: After dinner when Dale and Brennen are walking upstairs and Dale is explaining to him the "rules of the house", he explains that the house was built in 1825 by General Custer. George Armstrong Custer ("General Custer") wasn't born until 1839. (00:11:20)
Corrected entry: Dale and Brennan receive their Chewbacca masks near the end of the movie while they are in their "treehouse boat. But earlier in the movie the two are having a conversation in their room and Dale is wearing the mask already.
Correction: Dale had kept his mask through the whole movie. When he moved out of his dad's house, it shows him in his own apartment one day doing his taxes while wearing the Chewbacca mask. If he had actually ended up giving it away when he moved out, and had his parents buy a new one for him at the end of the movie, I would think that they would have bought both Dale and Brennan the exact same mask, so that one brother wouldn't get jealous if other brothers mask was better quality. Brennan's is different and cheaper, and he actually says that "It's OK that mine isn't movie quality" (unlike Dale's) to everyone there.
Corrected entry: In the Christmas scene, Brennan's Mother tells him that Nancy called and said that she was not his girlfriend, only his therapist. Later at the Catalina party, he calls her Denise.
Correction: Actually, his mother calls her Denise. Nancy is Brennan's mother's name.
Corrected entry: In the bedroom, Brennan's bedding is striped, while Dale's is plaid. When they combine the beds to form a bunk bed, the bedding has been switched and Dale climbs into the striped bed while Brennan is in the plaid bed.
Correction: But Brennan and Dale could have switched the beds themselves, just because they wanted to. Which would make that a character mistake. Building the bunk beds was something they did on a whim, so they most likely wouldn't get hung up on having to sleep in the bed that was originally theirs.
Corrected entry: Toward the end of the film when Derek flashes back to Brennan helping him fly a kite, the two little boys are roughly the same age - about 6 or 7. They are 3 years apart, Brennan would be quite a bit taller at that age.
Correction: The actor playing 9-year old Brennan (Bryce Hurless) was in fact 9 years old at the time (or close to turning 9 depending on when his scene was shot), while the actor playing 6-year old Derek was 7 at the time. While in real life, Hurless isn't very tall as an adult, children experience different growth rates, especially during puberty. So a short 9-year old that looks 6-7 to you could grow up to be 6'3", which I feel was part of the reason they cast the young actors as they did.
Correction: This is actually part of the humor of that scene, not actually a mistake.