Other mistake: Throughout the movie there are mistakes concerning the position of the star towards the turtle and towards the people on the Discworld. When the turtle is flying directly towards the star, then it should be seen by the people on the horizon and not close to the zenith. Or - at the end - when everybody is watching the babyturtles hatch out, A'tuin turns around. But the people are still watching the phenomenon looking in the same direction as before.
Continuity mistake: When Rincewind and Twoflower are at the Edge, Rincewind tells how he only knows the one spell from the Octavo and how he got it. In several shots of Twoflower the clouds in the background change drastically, often between one- and two-seconds shots of Rincewind. The clouds are almost motionless in every shot, so they could not have drifted that fast so suddenly.