I've got an Oyster monthly pass, so no problem there.
My gf has an Oyster pre-pay, since she doesn't live in London.

On Saturday, we were going from Central London back to Walthamstow and
since the Victoria Line has the (almost permanent!) hiatus in the middle
at the moment, I suggested getting the One from Liverpool Street to
Walthamstow.

When we got to St James's Street, I suddenly hit upon the bright idea of
getting off there, so I could walk to Sainsbury's on the way home.

As soon as we got on the platform and off the train, I remembered that
although PrePay is valid Walthamstow to Liverpool Street, it's NOT
actually valid to or from any intermediate stations so her Prepay would
almost certainly have some problem.

In the end, she walked all the way up to Walthamstow herself and touched
out to resolve the journey.

What would have been taken off her prepay if she hadn't taken the 15
minute walk to Walthamstow to do this?

There are no readers at St James's Street and the lack of any ticket
machines that offer Oyster meant that there was no way of checking
(since the ticket office is ALWAYS either closed or has a huge queue).