In message , Kat
writes
If you have an Oyster season ticket with no Pre Pay on it, 57 will show
on the POD and you'll just be stopped at the gates and directed to the
Assistance Window to pay the difference if you go out of zone.

I may have caused some confusion here. I'm assuming that a season ticket
which has never had any Pre Pay on it will do this.
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.
--
Kat "bears, said the taxi driver, is sixpence extra,
sticky bears is ninepence"