A leopard can't change his shorts
The phrase a leopard can't change its shorts appears many times in the Discworld series of books. It appears especially often in Unseen Academicals, as if Terry Pratchett is trying to make some special or unique points connected with the plot. Certainly the occurrence of this phrase in Unseen Academicals must add up to well over a dozen repetitions of the same joke, as opposed to only once on Soul Music, for instance.
Uses in other novels:
Snuff: "It has been known, as people put it, for the leopard to change its shorts." Vetinari's final meeting with Vimes.
Raising Steam: multiple times by Vetinari, Vimes and others in referring to Moist Von Lipwig's character arch (also: 'proof that a leopard can change his shorts')