Most scenes involving the playing of guitar are unsynced. The most noticeable is the "Beezleboss (Final Showdown)" scene when Kage starts playing the audio has changes in the guitar chords, but KG's hands aren't moving at all.
When JB and KG borrow the car from their friend, it clearly has stickers on it advertising the pizza business. During the car chase scene, the stickers are completely non-visible. (The reason there are no stickers visible in the car chase is because the shot of them falling off was cut. This can be seen in the deleted scenes on the DVD).
As Jack and Kyle are walking up the "Guitarway to Heaven" in the Rock Museum, they are at first walking completely in sync. Cut to a close-up of their feet, and they are out of sync. They are still walking directly next to each other, so it is not due to one of them changing their pace. In the next shot, they are once again in sync. When they reach the top of the stairs, out of sync again.
When JB meets KG playing the guitar on the street, JB is holding the guitar first in his hands but in the next shot it's on his back and again in the next shot he's holding the guitar in his hands but in the next shot he has the guitar on his back again.
After the first concert, back in the apartment, JB draws a perfectly straight pentacle with ketchup on the floor but in the next shot, one of the lines is crooked and the tape recorder has changed place.
When young JB leaves home and a map is shown with an animated line following his travels, the town "Charlotte" appears on the map twice, in both North Carolina and South Carolina. There is no Charlotte, South Carolina, the place marked on the map as "Charlotte" in SC is about where Greenville, SC should be. All other towns marked on the map of the country are correct.
In the showdown with the Devil, after JB and Kyle challenge him to a rock off, and he starts to scream "FUCK!", he turns around and you can see a white strap of elastic that holds some of the hair on his lower half on.
When KG and JB are driving, especially during the chase scene, it is very obvious that the car is on a trailer and being towed and not actually being driven. When the background shows that the car is pulling over to the curb, or turning, the wheel stays straight and does not rotate.
Ben Stiller's character explains how Satan must remain in Hell until he is reunited with his tooth (AKA the Pick of Destiny). Therefore, Satan could not have been on Earth, posing as Paul F. Tompkins' character, as he was not joined with his tooth until he left Hell, which would violate the rules. But then again - since when did Satan start playing by the rules?