http://news.revolut.com/post/146212645837/touch-it-tap-it-re...
I've been charged a wrong amount by TfL a few times in the past, I contacted them (either on the phone or via the webform) and I was always reimbursed. There are basically two situations where you get charged a wrong amount:
1. common: the "touch out" did not work. Just log in and click "refund previous trip".
2. rare: some weird software exception where your trip is charged via a Zone that you did not travel through.
The second happened to me a few times but honestly if I think of the massive amount of complex/arcane/exceptional software rules they must have ...I feel I cannot blame their software engineers :)