This obviously deserves a specialness award, even by my high standards of upgoofery. In my defence, I thought SouthEastern trains stopping at London Bridge was a fundamental truth of the universe, and I'm having seriously to reassess the world as I know it.
The train did go through London Bridge, and in fact paused there for several minutes; it was just that the damn doors didn't open because the service stops only at Blackheath and Waterloo East (yet takes twice as long to do it as the one that potters along via Woolwich Dockyard, Maze Hill, Uncle Tom Cobleigh and all).
It was no consolation to hear from the ticket inspector that I was the fourth person to emerge from the train and enquire sheepishly after the next service back to London Bridge; that just made me stupid without even being special. But he did call me 'Madam', and I don't get that often, so the extra twenty minutes' journey time wasn't entirely wasted.