We learn at the beginning that Brennan is 39. Later, during a bedroom scene with Dale and Nancy, we learn that Brennan was 17 when his brother, Derek, won a talent show by lip-synching "Ice Ice Baby." If the movie takes place in 2008, then the talent show was in 1986. "Ice Ice Baby" was released in 1990.
When Brennan is showing Dale how good a singer he is, a samurai sword is on the floor between them. When Brennan stops singing and stands up, the camera angle changes. When it goes back to Dale, the sword is gone.
During the first schoolyard showdown, we see the 3 main antagonists, 2 boys and a girl, walking towards Brennan and Dale. As they approach, the girl is on the left, but when they arrive to confront the step-brothers, the girl is now to the right of the boys.
Brennan is in Randy's office asking to be given the task of the Catalina Wine Mixer. Derek comes in and says to Randy, "Did you tell him the deal?", obviously making it clear he already knew that Brennan was given the job. However, after hearing Brennan out, Derek then decides to give Brennan the job. (In the DVD extras, this goof is explained because an alternate take (entitled "Catalina Heart Attack") shows Randy giving the job to Brennan without Brennan needing to ask for it, or repeat his request to Derek. Then Derek comes in and says the same line.)
From the time when Brennan is hiding in the tree-house during dinner to when Robert is consoling Brennan after the bunk beds collapse, Brennan's shirt changes throughout the scenes between a solid, light blue shirt to a striped blue and white shirt. This is most noticeable during the bunk-bed building scene and when Robert is consoling Brennan.
When Nancy moves in and first walks up to the house with Robert, they stop to greet Dale. In the next shot, Dale is shown still turning his head as if the couple keep walking a few seconds onwards. In the following shot, the couple are still standing where they stopped and the position of Dale's head is back to where he looked at them before the intermediate shot.
When Dale and Brennan are throwing Christmas presents while sleepwalking, only two smaller boxes land on the top of the bed. When they come back for round two, there are several larger presents stacked up at the foot of the bed.
When Derrick is talking to Robert about selling the house and Nancy is by the sink/dishwasher, Derrick's wife is standing by the microwave, holding her glass of wine. As Nancy is talking to them, there's no one there. The shot cuts back again to the guys, she's standing there again, and then back to Nancy and now Derrick's wife is standing by the dishwasher.
When Brennan is showing Dale the samurai sword with Randy Jackson's autograph his hands change position between shots. When Brennan is facing the camera he has both hands on it. During the shot over his shoulder he only has one hand on the sword.
In the scene where Brendan and Dale ask their parents for permission to build a bunk bed, there is a wide shot of the room showing everybody, and in this Robert's hand are by his side. In the next shot of Robert and Nancy, Robert's right hand is on his laptop.
After the sporting goods manager sarcastically hires Dale and Brennan he has his hands crossed when Dale begins to fart then in the next shot his fingers are on his desk and a few shots later his hands are again crossed.
After the drive home following the Prestige Worldwide presentation, the boys burst into the house. A crew member in black clothing, trying to stay out of the shot, is reflected in a mirror beside the main entry door.
When Derek comes over for Christmas dinner, he mentions that he was driving around with his buddy in a "brand-new" Testarossa. Ferrari stopped production of their Testarossa sports car in 1996, so Derek could not possibly be driving a brand new one. But, since Derek takes such pride in his accomplishments and the people he knows, it's possible that by the time he was 24 years old (his age in 1996) he could already be established enough to know someone with a brand new Testarossa. Derek never states what year this particular story takes place.
At the Catalina Wine Mixer, Dale plays Boats n' Hoes. While he is on stage, Brennan is still talking to Robert. In one shot though, Brennan is talking to Robert, and no one is at the drum set, but Boats n' Hoes is still playing and Dale is still singing.
The goof item below may give away important plot points.
When Brennan is singing at the Wine Mixer, he holds a note that shatters the glass helicopter sculpture. At the end of the song, when his mom and stepdad are smiling, the sculpture is intact in the background.