In message
, at
14:11:42 on Sun, 8 Aug 2010, CJB remarked:
TfL explained that it could not be aware of any over-charging until
the issue was reported by a passenger.

Apart, of course, from looking for cards where there were two top-up
[bank charges] on the same day.

Yes - some passengers will do that, but that doesn't actually matter.
What you are looking for is top-ups where there were two debits but only
one amount of credit added.
--
Roland Perry