Thief Of Time is a pretty good explanation of all the inconsistencies on the Discworld. Although I think the philosophers living for centuries is funnier.
Fred: No, look! Small Gods and Pyramids need to be set in the same approximate timeframe.
Bob: That's not true! That's impossible! The Omnians specifically take a couple of centuries to develop their pamphlet-based religious model as appears in all the books after Small Gods.
Fred: But that would imply that the Ephebian philosophers have lived through all those centuries - extremely infeasible.
Jane: It's really quite simple. As explained in Thief Of Time, when the glass clock was constructed it shattered time into pieces.
Jane: Much like the pieces of a jigsaw puzzle.
Jane: They start inside a box, and when you tip them out onto the table almost exactly half of them are upside down.
Bob: Is there a... point to this metaphor?
Jane: Raise your hand if you have a question, Mr. Miles.
Jane: Also, it's easier to put them together if you find the corners first, and looking at the picture on the box is very helpful.
Jane: It can take days to put together the ones with lots of pieces, and... sorry, what were we talking about?
Fred: If I had understood a word of that, I'd probably be annoyed that you spoiled the book.
Jane: Hand, Mr. Phillips. Ten points from Gryffindor.