You may or may not notice a more subtle thing going on in London. The turnstiles used for the railways and Tube work on simple reflective sensors to see who has gone through before closing again. If you cover the sensors, it appears to the barrier that you are still passing through, so you can get as many people through as you like.

It's rather easy to do: I've used it in the past when my wife had a non-magnetic ticket, and the station staff couldn't be bothered to come and let us through.