The act of passing on Doctor Who's lead role is a weird one, since the actors are stuck in a limbo where they know they're on the way out, but they don't know who's going to take their place yet. Peter Capaldi however managed to figure out there was a monumental change coming for his replacement... thanks to a pair of trousers.
Speaking at his final New York Comic Con panel as the incumbent Doctor, Capaldi revealed that he accidentally learned that the next Doctor would be female a few days before he was supposed to, thanks to, weirdly enough, a visit to his tailor:
I went into Paul Smiths, which is a very wonderful clothes shop in London where I buy my suits, and everybody knows me in there. And they said, "We just got a call," they said, "from the Doctor Who office saying, 'Can we have a pair of [Peter's] trousers, but with a waist size 30?'"
And I thought, 'I'm finished, I don't need any more trousers!' — because Doctor Who wears Paul Smith trousers, for the fashionistas among you — so they were obviously looking for a pair of trousers for the new Doctor Who to wear. And I thought, 'Well, that can't really be a man with a 30-inch waist. That must be a lady then!'
Capaldi got confirmation of his hunch a few days later, when he was called days before the announcement that Jodie Whittaker would become the show's first female Doctor — and found out after calling the actress that she'd moved into a place a short walk away from him to prepare for filming Who in Cardiff, but was unable to say hello without revealing she was his replacement.
So there you have it: One of the biggest changes in Doctor Who history, nearly done in by a pair of pricey trousers.