After Seth dances with the period blood girl, she goes away and he tries to stash a bottle in his pocket. When he goes to put the bottle in the pocket, we see the blood stain is not on his pants yet.
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.
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.
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".
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 takes the gun from the officer to shoot at the car, the safety is on, you can't shoot with the safety on.
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.
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.)
When Seth is running through backyards with the laundry detergent bottle supposedly full of beer, it is obviously empty, such as when he throws it over the fence.
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 fake ID features no indication that "McLovin" is an organ donor.
In the end credit music listings for Motorhead's Ace of Spades, Ian "Lemmy" Kilmister's last name is misspelled as Kilminster.
When Fogell first pulls out the fake ID from his pocket to show it to Seth and Evan, you can clearly see the ID is as thin as paper and laminated with sharp, transparent edges. When the close up is shown a few seconds later, it looks more like a basic ID card.
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.
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.
During the scene where Evan is unintentionally cursing Becca over the phone, a boom mic's shadow is visible on the asphalt behind him.
The clerk at the liquor store dials a long telephone number when calling the police instead of just three digits for 911.
When Evan is playing video games he comments on the use of an M16 assault rifle. The games he's playing is The Getaway: Black Monday (2004), a British police game where M16s are not used.