This article is from the Pratchett Newsgroups FAQ, by Orin Thomas email@example.com with numerous contributions by others.
Well, he is fallible. Known inconsistencies are documented in the
Annotated Pratchett File. (available at the Pratchett Archives).
The Author suggests, however, that there are few inconsistencies
merely several alternative pasts. :-). These are getting rarer
as time goes on and a discworld "lore" builds itself up.