The following is a hypothetical scenario, since I do not have Oyster of
any sort.

If I have Oyster pre-pay, and I get on NR at Wandsworth Town (no
barriers of any sort) and emerge at Waterloo, 99.99% time it will be a
free journey, since rarely do I see ticket collectors there.

If, On the 0.001% occasion there is a ticket inspector there, what will
he do? If I have a valid credited Oyster card, presumably he will have
to find a way of debiting it for the correct fare which, through no
fault of mine, I had been unable to do at Wandsworth Town.

Answers please!

Marc.