I’ve been re-reading the HP books and I got to Prisoner of Azkaban (be still my beating heart, it is still the best one) and, at Christmas there’s only 12 people around the dinner table. Trelawny 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 die!!!!!
Ron had Scabbers in his pocket.
Which means that there wasn’t 12 people when Trelawny arrived. There was already 13.
And Dumbledore stood up.