Christian Hansen wrote:

My deduction is that perhaps (and don't try this at home, folks, unless you're
willing to lose lots of money) when touching in at an NR station in a zone for
which you have a valid travelcard on the Oyster, then not touching out at a
station which doesn't have a reader, you won't be charged.

Yep - on the rare occasions I take WAGN at Walthamstow to Liverpool
Street instead of the tube on my 1-3 Oyster, I nearly always forget to
touch in/out at the Walthamstow end - and nothing has happened yet,
except an unresolved journey that doesn't actually stop me from exiting
any gates.

What WAS annoying though was once when I tried to exit some Tube gates
and couldn't, not knowing the Travelcard on the Oyster had expired and I
had been eating away my Prepay for 3 days all the way down to nothing.
What happened to those "TICKET 1 NEAR EXPIRY" messages the gates used to
say - they don't seem to do it any more.