On the Unrated version, when director Sean Anders is talking at the very beginning of the movie, he says "these tits are good," but his mouth doesn't move. (This was possibly done purposely, as the intro sequence was intended to be rather absurd.)
In the Scene after Rex opens the garage door to see if the Judge is gone, he starts smashing the garage door. In one scene the bottom of the door is bent inwards, the next it's not, then it is again, and finally when he knocks it off the runners, the bottom is like new again.
When Ian and Lance get arrested by the police and put into jail, Lance has to give the police his cell phone and watch (and his handcuffs). Ian is supposed to do the same, but when they come in to the arrest, Ian still has his watch on.
When Lance and Ezekial are in the horse wagon, you see Lance's arm shift between shots. One time you see around Ezekial then his arm is on his side (or somewhere in the vicinity but not around Ezekial). It does shift a few times.
In the scene when Ian misses the laundry basket with his down-jizzed underpants, you can clearly see that his underwear lands in a position in front of the basket. But when Rex is coming in to the room, the pants have mysteriously moved nearer Rex's foot and are now beside the laundry basket.
In the scene when the trio is at Brandy's home and Rick is coming for Brandy, he's stopped by Felicia just in front of the bedroom door. Then when Ian moves Felicia away from the door he takes a picture with him and it falls to the ground. But later, when Lance is stuck to the front door with a part of the bed and Rick is trying to grab his face, you can clearly see that the picture is up on the wall again without anyone touching it.
When the shot cuts to Ian swearing after Ezekiel tells him that he blew his head gasket, you can see a girl in the background dressed in very modern clothes, yet they're supposed to be in an Amish village where the only people in modern clothes are Ian, Felicia, and Lance. Then a second later when the shot cuts back to Ezekiel, then to Ian again, the girl has disappeared.
The passenger window on the green sports car during the shooting towards the end of the movie. Shortly after the car is shoot, you see the window closed. In the next shoot the window is open and the passenger clearly visible, but is closed afterward.
In the scene right after Ian and Felicia come across the tree with sneakers in it, Lance is putting glasses in a case and on the hood of the car. Ezekiel rides up in the carriage, and the glasses are suddenly outside of the case, sitting next to them on the hood.
When Ian comes home after Lance's party, his little brother Dylan asks him whether he wants to smell his finger. In the first shot (right hand finger), Dylan's hair is still long. In the second shot (left hand finger), it's short.
In the video montage of the beginning of their trip (played to MGMT's "Time to Pretend") you actually see shots of Indianapolis before you see shots of a popcorn bin in Boone County, Indiana on the side of highway 65 south. This is incorrect because traveling south through Indiana, you actually would go through Boone County before Marion county, where Indianapolis is located.
Although the beginning of the film is set in suburban Chicago, when Rex is driving Ian to work at the mall, palm trees clearly are visible in the background immediately before a jump cut to the mall. (The movie was filmed almost entirely in south Florida.)
Seth Green's character when he finds the GTO stranded on the road mentions that it is a 1969 GTO with a 455 HO engine. The 455 was not an option till 1970. The largest factory engine the GTO could have in 1969 was the 400.
In the bathroom when Ian unknowingly reaches for toilet paper and touches what presumably is the genitals of the man in the adjacent stall, a light-pink prosthetic is clearly seen under the toilet paper.