When Rincewind and Twoflower are on the rock next to the Circumfence, the string holding Rincewind's hat around his neck keeps moving from above his beard to below it between shots.
When Rincewind and Twoflower are tied back-to-back, Rincewind says that "if complete and utter chaos was lightning, then you [Twoflower] would be the sort to stand on a hilltop in a thunderstorm wearing wet copper armor shouting 'All gods are idiots'."; you can tell by the way his mouth moves, however, that he actually says 'All gods are bastards', which is consistent with how it is worded in the novel.
The book and the notes about the secret passageway say to pull the third book from the right but the one that is pulled is nowhere near the third book from the right (or even the third from the left).
In one scene, Twoflower is offered a dagger, which he takes by the blade.