[robjan]

In London, the barriers automatically charge the maximum fare if you forget to touch in or out of the system. It helps because you can jump the barrier to get into the system but you never know if ticket inspectors/security will be present at the station where you plan on alighting. The penalty for being caught jumping the barrier is significantly higher.

[thaumasiotes]

Caltrain (the heavily dysfunctional system serving a minor portion of the San Francisco Bay Area) is similar, but a little less convenient --

Tag in and forget to tag out, you pay the maximum fare.

Forget to tag in, hope you don't get checked. There is no option to tag out without having tagged in. (Although I've seen this happen to someone, and he was just given a warning.)

Get checked without having tagged in, pay $400, which is about 30 times the maximum fare.