[Michael Hoffman]
If you don't touch out and then back in again at Canary Wharf, you could
be done for fare evasion.

[XmaX]
I suppose you're right, but how can anybody know that I am evading?
OK, it might look a bit strange that I went there and then I go back,
but what's wrong with it? I might have just got a phone call to go
back, or went to meet someone at the station, or just wanted to visit
every single DLR station. Nothing illegeal so far, and nobody can
prove me anything, unless they check CCTV.

Well you'll have your shopping on you, so that's one way.

Another way is if you were caught not touching in or out by an
undercover inspector. Or if, when questioned on the train about your
anomalous touch-in, a witness stepped up. Or they could have CCTV tapes
checked if there are any.
--
Michael Hoffman