Kat wrote
[...]
An Oyster which has inadequate Pre Pay or none because it has been
used
up will open the gates but not allow you to use your season ticket
again
(even if it's perfectly valid) until the amount owing for the
extension
has been cleared.
I can see this causing major problems.

AND you are assuming your exit station is staffed so you can pay if you
wish.

The major problems would be if it is unstaffed and you get 'refused'
the next morning.

And what happens if the next use is again at an unstaffed station ?

--
Mike D