Mrs. Proust makes a comment about preparing medication for Sergeant Angua's "hardpad", which leads Tiffany to realize Angua's a werewolf. Hardpad is an older term for canine distemper... a viral infection that is normally fatal to puppies and kills about 50% of the older dogs that catch it. It not only causes problems with a dog's paws, but also with the entire digestive system. Poor Angua, the vegetarian werewolf by day and eater of Ankh Morpork's suspicious meat industry by night, is now paying the price for being a city werewolf.
You (Granny's cat, not YOU you) is described as blinking slowly at Tiffany, then looking away. This is an extremely high compliment in Feline; of course Pratchett the Kindhearted Cat Lover would not have had You doing this gesture by accident or mistaking it for disdain.