David Jackman wrote:

(snip quoted text)

The problems will occur at stations with no gateline - such as most of
the DLR - where it is all to easy to forget to touch out if you get
distracted at the wrong moment.

"Operator error" would be the euphemism used to describe this by an IT
support desk - but whilst that has the undertones of "stupid user can't
do the simplest things", it's is all too easy to imagine oneself being
caught out when in the situation you describe above.

Thus passengers need to be encouraged to change their behaviour, and
there should be lots of signs and other visual clues to remind for
passengers who've just got off a train that they need to touch out. At
Finsbury Park (an ungated station) the wall of the tunnel walkway
surrounding the Oyster readers is painted bright yellow - things like
that should be par for the course.