Tim Woodall wrote:

Then there's sharing a card to only have one cap. But I wonder how
many people are going to make a journey, then phone their accomplice
"Ok, I'm out. Now you make the journey." It's the sort of thing some
university students might do to prove it can be done but it seems
unlikely there are many other people who will bother.

There were people who shared paper travelcards by hiding them on the system,
for instance someone who commutes from zone 6 to zone one sharing tickets
with someone who commutes from zone 1 to zone 6. I know because I saw one of
them hiding his ticket.