The Dartford Crossing.
More specifically, the new toll system they've introduced. It's a massive, massive con.
Instead of just deducting the cost of the toll from a registered bank account, it requires you to add money to an account, which it will then deduct after each crossing. Now - they have an auto top-up system, which will replenish your account if you get too low. The lower limit is £10. And minimum top-up is £10. So the moment you drop below £10, it'll top you up with £10, meaning they've always got at least £10 of your money at any one time.
The most obvious method would be to top you up the moment you drop below £0, but that would give them no scope to fleece you, would it?!