Tube Ticket Touts
At many major Tube stations you'll find people at the exits or even inside the station offering to sell you a cheap Travelcard, or asking if you can give them one that you've finished with so they can sell it on.
Needless to say, this is antisocial (touts have been known to get violent with people) and, well, it's cheating. There's also some evidence, apparently, that profits from touting can go to local crime.
Underground staff have been instructed to come down hard, both on touts, and on people obtaining their tickets from them. You may also hear announcements warning you of touts; these can be heard fairly often in Leicester Square Station, for example.
As a counterpoint, you could always give away your Travelcard.