Odd happening to me last week. I was going to Shortlands from Elephant and
Castle NR station. I have an Oystercard with an annual Zone 1-2 ticket and
£10
of prepay on it for out-of-zone travel.

I saw the Oyster reader in the E&C station and thought that meant that
Shortlands had one too (like many NR stations), so I touched in, assuming
that
the fare would be deducted from the prepay when I exited.

Of course, there was no reader at Shortlands and I thought "Well, I'm
going to
lose the maximum fare as I didn't touch out."

Yesterday I went to add some more money and discovered that nothing had
been
deducted from my Oystercard prepay amount.

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.

The system is set up so that if you "touch in" at any station (tube, NR, DLR
or joint) with an Oyster that has a valid travelcard for the zone the
starting station is in, and do not touch out at the end of your journey, you
will not be charged.

This protects someone with, for example, a Zone 2&3 travelcard travelling
from Mile End tube to Maryland NR (changing at Stratford where there is a
cross platform interchange) from being overcharged as they would not be able
"touch out" at Maryland (national rail, no barriers) but would have "touched
in" at Mile End.