During the Harry Potter party Carl's lightning scar changes position.
When Carl is avoiding Peter's phone-calls, he leaves the cell phone on the arm of the couch. After Peter rings Carl's doorbell, Carl's cell phone is now with him.
As Carl and Allison enter Norman's party, Allison is wearing high heels but, after when they climb over the fence to get into the arena she is wearing a different pair of shoes.
The first time Carl walks out his door, there is no nameplate there. The next time he is at his door, the name "Allen" is there. This occurs several times throughout the movie.
After he saves the man from the ledge, the camera shows crowd cheering and a fireman is standing by Lucy, but in the close up, it's another guy.
In the scene where they travel to Nebraska and attend a football game and their friends see them on TV, the bottom of the screen reads "Nebraska vs. Oklahoma". It should read the other way around; as they were in Lincoln at a home game; the home team appears second.
The end credits claim to list the cast in order of appearance, however Zooey Deschanel is listed second despite appearing later than other actors listed below her.
When the LDS missionaries come to Carl's door, neither of the missionaries are wearing Name tags. Latter Day Saint missionaries always have name tags identifying them as Elders of the LDS church.
Carl and Allison take cover from the rain in a barn. Carl, who should be soaking wet, has only a few water stains on his shoulders. His hair and face are completely dry.
When Carl is in the hospital and asks what time it is, he is told 5:30am. However, you can tell that the sun is up through the window behind him.
In the scene where Carl is singing and playing guitar to the jumper out of the apartment window, the adjacent window can be seen to have a thick curtain from the outside where it appears to be much thinner from the view inside the apartment. Also, this same adjacent window can be clearly seen from the inside of the room although at times it is blocked off with a solid wall.
When Carl is filling up the gas container, the read out on the gas pump shows that he filled more than 5 gallons when in reality he does not have a 5 gallon container.
When Carl is racing Lee's motorbike to get to Allison, Carl is not the one riding the bike. When he is on the freeway he can be seen holding in the clutch even though the bike does not shift gears.
When the handheld-keyboard player in the band Munchausen by Proxy plays the passage from "The Star-Spangled Banner," the audio plays it in C major, but her fingers on the keyboard play all white keys with the melodic line resolving on an A, following a pattern that would in reality modulate the melody to A minor.
In the scene where Carl is singing and playing guitar to the jumper out of the apartment window, the singing is out of sync with his mouth during one verse.
A video store section labeled "Family" contains R or NC-17 rated horror films such as The Thing (1982) and Saw (2004).
When Carl's car runs out of gas, he has already turned the car around, so that it was facing downhill. The obvious move had been to simply put the car into neutral (N) and let it ride down the hill to minimize the walking distance to the gas station.
After Carl goes bungee jumping he is seen answering his cellphone while still suspended from the cord. Due to the risk of them falling out of your pocket mid jump, most, if not all, bungee services would advise you to empty your pockets of anything valuable, like a cell phone.
When they are riding on the scooter, both the first time with the gas can and the second time he is driving, they go through several red lights. This is why car horns are blaring throughout the scene.
When Carl is singing the song "Jumper" to the man on the ledge, he sings it incorrectly. He sings "And if you never want to see me again, I would understand." The actual lyric is "And if you do not want to see me again, I would understand."