Last night, I didn't touch out of the tube on the Waterloo and City line (easily done at Waterloo, I might add) - TFL charged me £6.40. I realised later on, so went to a ticket window and got it sorted (very easily - no questions asked).
Checking the TFL website, they can charge up to £7.40 for not touching out (when did that happen? Didn't it used to be £4?). Seems a bit steep to me - it wasn't so long ago the penalty fare was £10. One to watch out for, and leaves we wondering why TFL need us to touch out, quite so much...?