It may have been corrected in later books, but in the Audio recording of Nightwatch, the performer reads a conversation between young Vetinari and his aunt on what would be page 195 of the hardcover version; He refers to Vimes as 'Vimes' instead of 'Keel' the name Vimes is using in the past. In the book it is 'Keel' but Vetinari would not have known Keel was actually called Vimes.