Incomplete Journey

From Frikipedia

The most heinous of all crimes on the London Underground. Woe betide the foolish traveller that forgets to swipe in or out as they journey round the parts of the capital.

Despite the fact that the only real way to get an incomplete journey is to forget to swipe on one of the platforms at the overland parts of the network, where it is not essential in order to get in or out, effusive Underground staff will assume you jumped the barriers at some point (despite the fact that if you had, why did you bother swiping OUT at the other end?) and slap you with a bubbish fine.

Failure to pay the fine can either result in a trip to John Menzies, or repossession of your Oyster card without them returning the deposit.