Colum Mylod wrote in
:

Shirley (SCMS) the big O could allow immediate exit after entering?

I assume it's some kind of revenue protection thing along the lines of a
paper ticket not working entry barriers at the same station twice within
15 minutes. That was obviously designed to stop people passing their
ticket over the barrier to a friend waiting outside, so the problems with
Oyster working properly on immediate exit at the entry station would seem
to be of a similar nature.

Iain