When Evan and Seth are first looking at Fogell's fake ID, you can clearly see that there is no black magnetic strip on the back. When the police are looking at it in the liquor store the magnetic strip is visible.
The signature on Fogell's fake ID is supposed to be written as "McLovin". When Seth and Evan see it, the signature does in fact say "McLovin", but when the police look at the fake ID, the signature is written as "McLoving".
One of the partygoers (Clark Duke) appears first at Jules' party when Seth enters with the liquor, walking behind Seth, and in the very next shot, he is in the backyard gazebo where Becca is waiting for Evan.
When Seth is holding the detergent bottles and arguing with Evan, you can clearly see that the dent on the front of the detergent bottle in his left hand keeps mysteriously disappearing and reappearing.
When Evan is at the first party and the six men enter the room to smoke or do cocaine, the man in the blue track jacket notices Evan in the corner and asks, "Who's that guy?" At that time he is holding a vodka bottle in his left hand and points at Evan with his right hand, which is holding a cigarette. In the next frame, when everyone turns to look at Evan, the cigarette is now in the man's left hand, and his right hand is empty. Also, the bearded man in the white hat appears to be missing the cigarette that he was just lighting seconds before.
At the start of the movie, when Seth and Evan are in the store, you can see Evan filling up a slushy cup and in the next few cut scenes, his hand retains the shape of a slushy cup, but there is no cup in his hand.
When Mark first confronts Francis at the party he takes the phone out of Francis' hand, the screen switches to a shot of Seth, and when it comes back to Mark and Francis all of a sudden the phone is back on the wall.
When Seth is ranting to his Home Ec teacher about being left alone at his station, the shot from behind his shoulder facing his teacher it appears to show Seth in a yellow shirt however this is simply the lighting on the shirt making it look paler.
When Seth is hit by the cops, the cops yell at Seth and Evan to lay down in the street. When the camera is filming them from the side you see that their legs are crossed over each other's, but when they are filmed from the front their legs are touching but not crossed. They switched back an forth between crossed and uncrossed throughout the scene.
When Evan and Seth are first approaching the school the sign says there are 2 weeks left of school for seniors. When confronting the Home Ec teacher in the beginning, Seth claims there are 3 weeks left in the school year. Later in shop class, Evan says it's the last 2 weeks of school.
When Fogell is riding in the back of the police car, the second time they run a red light, headlights of a car close behind them are shown in the immediate next scene, when the camera is in front of the car, looking at Fogell's face. If they really had run the red light, there would be no cars behind them for a while.
When Slater crashes the patrol car into the cement piling, we see the lightpole break off and hear it hit the ground yet in the very next frame the lightpole is back on the piling and is now just dropping sparks.
During the second fight, Mark throws a bottle at Seth, which ends up hitting the cocaine guy. He bends over grabbing his head with both hands and screaming. But in the very next shot he is standing still like nothing happened.
When Evan and Seth are talking to the creepy nerd guy outside the liquor store after Seth gets hit by the nerd's car, the "open" sign behind the nerd guy can clearly be seen changed from off to on from shot to shot.
At the very first shot of of the opening scene where you see Seth driving in his car and using his mobile phone, his seat-belt is twisted at least once. In the next close-up shot and the following wide shots, the seat-belt, however, is not twisted anymore.
After Seth gets hit by the car in front of the liquor store, when the driver of the car pulls out his wallet and offers them money, the wallet goes from being in his left hand to being in his right pocket and back again between shots.
When Seth and Evan are entering the party accompanied by the weird guy, a guy smoking from a pipe can be seen. However, no smoke is visible inside the pipe while he is inhaling. Also, he seems to talk right afterwards but he does not exhale any smoke.
In the scene outside of the grocery store where Fogell works, the license plates on the fronts of the cars are California plates. The license plates on the back of the cars, on the buses, and other vehicles are not from any state and don't have a state name on them.
When Fogell (McLovin) is shooting at the burning police car near the end of the movie, he fires the gun 10 times. The gun used is a Smith & Wesson 4506, which uses a 8 round box magazine; with the magazine and one bullet in the chamber, he would only have 9 shots.
When Fogell's fake ID is shown, it clearly shows the birth date as 1981, and throughout the movie he is referred to as being 25. This is more or less in line with the movie being made in 2007; the 2008 date on the license that would seem to make him older is the expiration date.
Towards the end of the film, Officer Slater has McLovin sign a form saying that after McLovin was punched in the liquor store, someone stole the cop car and trashed it, which would relieve the officers of their responsibility. However, it would not explain why it took them five hours to report it.
When Young Becca is looking at the penis drawing, the drawing itself obviously changes from the "in front of Becca" to the "behind Becca where we see the drawing" scene. (Having the young actress actually view the penis drawing would have violated California law. In the "behind Becca where we see the drawing" scene, a small-handed adult is actually holding the drawing.)
The "Green beer" that the main characters bring to the party is obviously a mixture of beer and detergent. In actuality even a small amount of detergent would have caused the beer to be really foamy and disgusting. The "Green beer" would have been unbearable to drink and would have made anyone who drank it sick.
The goof items below may give away important plot points.
When the bottle of Goldslick Vodka breaks on the bus, you see broken glass and liquid spilling out. When they show the closeup, with Evan's hands nearby, it just shows glass bouncing around and while there is liquid on the ground it is not constant with the amount that would came from a bottle just breaking.
As McLovin is about to start shooting at the police car the gun does not have the hammer back and he starts shooting it without cocking it, however the gun is single action/double action and does not need to be cocked prior to firing.