Continuity mistake: In the scene where Grug is chasing after Guy, they both wind up stuck in a tar pit. After successfully escaping they are covered in tar and Grug even gets himself stuck to Guy. In the next scene everyone is running from the exploding ground up between the mountains and both Grug and Guy are completely clean and free of tar.
Continuity mistake: At one point, Grug gets bitten in the bum. It's clear that his undergarments are made from lion skin. Yet in the next shot where he's climbing down, his undergarments are now just plain gray.