I've been re-reading the HP Books and I got to the Prisoner of Azkaban and, at Christmas there's only 12 people around the dinner table. Professor Trelawney comes along and Dumbledore stands up to draw her a chair. She freaks out, 'I must not! When 13 people dine together, the first to rise is the first to Except... Ron had Scabbers in his pocket. Which means that there wasn't 12 people when Trelawney arrived. There was already 13. And Dumbledore stood up.