Continuity mistake: When JB dreams about singing "Master Exploder" in the small bar, he points at one guy in the audience and literally blows his mind. When he starts pointing you can not only see JB's face, but also how his elbow is in a slight angle. In the next shot the camera shows the guy JB is pointing at and his elbow is completely stretched. The views change several times and the angle of his elbow changes after every cut, until the guy's brain is finally blown out.
Continuity mistake: As Jack and Kyle are walking up the "Guitarway to Heaven" in the Rock Museum, they vary between marching completely in synch with each other and out of synch. They are still walking directly next to each other, so it is not due to one of them changing their pace.
Revealing mistake: When JB and KG are doing the powerslide in the Rock N Roll History Museum, you can see that KG is wearing knee pads to help him slide.
Continuity mistake: When JB is in Hollywood for the first time and the guy plays Bach, you can see two Goths walk past and in the next shot the same guys walk past again.
Continuity mistake: When JB and KG are powersliding under the gate in the Rock n' Roll history museum, in one of the shots the gate is so far down that they would not be able to slide under. In the next shot the gate has moved up. (01:07:00 - 01:12:00)
Continuity mistake: During the car chase scene you can notice multiple stickers on the doors of Lee's car, but as soon as they show Lee watching the news, the stickers disappear in the subsequent shots of the chase.
Continuity mistake: When Kyle tells Jack that once he can get the remote, he will then be able to choose the channel. The position of the remote changes 90 degrees instantly. (00:18:30)
Continuity mistake: The scratch marks left by Satan at the end change length instantly. (01:22:20)
Visible crew/equipment: When the police are chasing Kyle and Jack, there is one shot where a lorry is heading right towards the camera. In this shot you can make out the camera equipment in the chrome bumper. (01:12:05)
Continuity mistake: In the diner scene JB drops his pen when KG goes over to the girls. Camera changes and JB has got his pen in his hand again. (00:44:30)
Other mistake: When JB is leaving the cafe, he pays money, but he did not buy anything.
Continuity mistake: Just after KG and JB swerve round the lorry, the lorry disappears from the road. It should be visible as it doesn't drive off the road. (01:12:10)
Revealing mistake: During the big chase, watch closely when the car goes over the ramp and does a flip. As soon as it lands, you can catch a brief glimpse of a partially-destroyed film camera (covered in black material) rolling along the ground. Its position on the road lines up with one of the previous shots, so it's definitely a camera. (It looks like the car landed on it and threw it along the ground).