Plot hole: At the wine mixer, where Dale is working as a chef , there is a band playing on stage. The drum kit used is the exact same drum kit (chimes and all) as Dale's, as seen in the house. If a band was hired to play they would have certainly brought their own equipment, and there is no way Dale would have brought his personal kit to the event, for somebody else to play. Dale ends up playing this drum kit anyway, when the band walks off and Brennan sings.

Factual error: A video is shown of Derek Huff (played by Adam Scott) singing "Ice Ice Baby" at a school talent show that took place when his brother Brennan (played by Will Ferrell) was 17. Brennan is 39 in the film, placing the talent show 22 years ago in 1986. The song "Ice Ice Baby", recorded by Vanilla Ice, came out in summer of 1990.

Continuity: When Dale is questioning Brennan as to why he is sweaty, and again when he comes downstairs after examining his drum kit, the remote control Brennan has swaps which hand it is in a few times and his hands keep switching their position too.

Visible crew/equipment: After their argument in the car (after Brennan and Dale have shown their presentation), Robert rushes into the house. As the camera moves backwards, you can see a crew member reflected in the mirror on the wall.

Continuity: When being attacked in the schoolyard, Dale and Brennan are in a different position relative to the big tree whenever the camera angle changes.

Revealing: In the scenes in which Brennan and Dale are fighting with the young children in the park, it is clear that in quite a few shots the children have been replaced with doubles.

Continuity: After the first dinner scene with the family, Dale finally decides to leave and head upstairs. He places his napkin down next to his plate. In the next shot it has vanished. It then reappears a few shots later.

Continuity: When Brennan and Dale first meet on the front lawn, they stand and stare at each other for a while. With every shot change, it continues to change from either being sunny or shadowy in the spot where Brennan stands.

Factual error: In the film, the Catalina wine festival supposedly takes place on Catalina Island. Yet in a couple of shots you actually see Catalina Island in the background. That's because they are actually at Trump golf course in Rancho Palos Verdes and the island is 26 miles across the sea.

Continuity: When Dale and Brennan first meet at the mixer, a lady with a big blue hat keeps appearing and then disappearing in the background.

Continuity: In the dinner scene where Derek is over, he is telling a story to Robert about fishing. During the whole scene if you watch Alice who is sitting beside Derek, her wine glass repeatedly jumps from her hands to back on the table instantly.

Continuity: In the christmas scene where they are decorating the tree, we see Robert eating a christmas-tree shaped biscuit. When he gets up, the biscuit is suddenly larger and more whole than before, where it was almost finished.

Continuity: Near the start when Nancy and Robert first meet and are undressing themselves in a hectic lust for one another, Robert removes his jacket in a wideshot and his shirt and tie are still on. But in the next closeup the tie has vanished and he is suddenly unbuttoning his shirt.

Continuity: When Nancy and Robert find the kitchen in a mess, after Brennan and Dale's sleepwalking episode, the mess on the floor and counter differs between shots.

Continuity: In the scene of the step brothers' first dinner together, Will Farell can be seen picking up a French fry, but in the next shot he is holding a half eaten chicken nugget.

Continuity: When Nancy and Robert are yelling at Brennan and Dale for fighting in the front garden (and ban television for a week) the food (green beans) on the plate in front of Dale keeps on moving between shots.

Continuity: When Brennan and Dale are in the tree house and Derek comes up to talk to them, the soda can over Derek's shoulder moves around between shots.

Continuity: The bottle of ketchup in the dinner table scene, early in the movie, alternates from right side up to upside down, from shot to shot.

Factual error: When Brennan Huff is finished singing at the wine festival, you see the helicopter ice sculpture smash, but when it zooms back to Brennan Huff, you see people holding glass cups. These should have broken with the ice sculpture.

Continuity: When Brennan and Dale are in the tree house and Brennan's brother comes up, Brennan has a magazine in his hand, and each time the shot changes, he doesn't. It alternates between the two.