MIG wrote:

It's a bit of a contrived example...

Perhaps, but I was considering a similar, "real" case (though only on
a theoretical basis) on the day of the Shoreditch last run. (Though I
didn't go in the end.)

Well, someone mentioned "trainspotters". What does happen if you don't
leave the station, but just sit with your egg sandwiches at Acton Town?
Or if you eventually leave, but outside of the two hours? Lots of
unresolved journeys?

Or what if you travel from Woodford just to meet someone and help with
their luggage when they get off the Piccadilly at Barons Court, before
travelling with them back to South Woodford?

There are many examples of not leaving the system that are sufficiently
possible that tens of people must do them every week.

Or here's a related one for that part of London. South Woodford to Newbury
Park can be done by changing at either Woodford (and travel through Zone 5)
or by changing at Leytonstone (and staying entirely in Zone 4). Either
involves a change - how does the system know which you've used? And also
when part of the Hainault loop is down at weekends, the "replacement"
service is travelling via the other way - can someone who doesn't have Zone
5 on their travelcard (and maybe not even pre pay - perhaps a paper)
legitimately travel via Roding Valley when they can't go via Wanstead?