A few weeks ago I got the wrong branch of the Northern Line coming southbound.
It didn't really matter, I got off at Bank and went on the Central Line to Tottenham Court Road and got to Waterloo that way (it was too late for the Waterloo and City Line).
But getting the wrong branch of the Northern Line northbound is more serious. Look at this story that I read on wikipedia yesterday, in the page for 'The game':
"Three young men, following a heavy night out in the west end, accidentally took the wrong branch on the northern line, and found themselves stranded at East Finchley. As they had no money, they decided to wait on the platform until the first morning service to take them back into town. To help the time pass, they decided to play a game, and one suggested that they should try to forget about where they were, and that they were playing a game, and the first one to think about the game, or about East Finchley, was the loser."
This has now been formulised into a game that apparently anyone can play . The object is simply to forget you are playing the game. You lose every time you remember that you are playing, and you should announce the fact that you have lost.
There are some good tactics for winning the game mentioned on the wikipedia site, liking putting post it notes up where your opponents can see them that say 'the game'. But this does cause philosophical problems as you were probably thinking about the game when you wrote the post it.
There is a wikipedia deletion page for the article where there is a debate going on about whether or not to delete the page