There is this rule in Oslo public transport that has me wondering. I am gonna get to the more extreme example in the title of the question near the end, but I will start with an actual example that happened to me today:
Today, I took the metro from the Jernbanetorget to Ullevål Stadion for a quick errand, and got a single use ticket for the trip. This ticket lasts for 60 minutes.
When my errand was done and I was about to hop on the metro for my 15 minute trip back home to Tøyen, I noticed that I still had 5 minutes left on my ticket from earlier. So my ticket would be valid for the first 5 minutes of my trip, but for the last 10 minutes of my trip, my ticket app would say that I had no valid ticket.
You may transfer to other means of transport (free transfer) in those zones for which the ticket applies, as long as the ticket remains valid when you get onboard.
I reasoned that my stop at Ullevål Stadion can be considered a free transfer, and that the ticket is considered valid for the entire trip home to Tøyen as well, as long as the ticket is valid at the time of boarding.
It only makes sense that a ticket is valid for an entire leg as long as it is valid at the time of boarding. There are many trips inside Zone 1 for which the fastest route takes longer than 1 hour (such as going from Bjørndal to Voksen Skog). If you could not go on such a journey on a single Zone 1 ticket of 60 minutes, but would have to purchase multiple tickets for a single trip just because the buses are slow, that would be rather unreasonable.
This makes me wonder, though, because there is no way to prove what sort of trip you have been on. (The ticket app does not collect location data, and there are also "offline" travel cards).
As an extreme example, I could have taken the bus or metro from somewhere else in Groruddalen to Grorud, then, with 2 minutes left on my ticket, boarded the 31 bus to Snarøya. Over an hour and nearly 40 stops later, when I am nearly at Snarøya, my ticket should (even though it technically expired over an hour ago), be valid, since it was valid when i boarded the bus.
However, if I am stopped by ticket inspectors, I can not prove this. I might be a travel enthusiast who likes going around Oslo for the views. However, to ticket inspectors it might seem like an unlikely scenario that I boarded the 31 bus with a ticket with only 2 minutes left on it, over an hour ago.
One solution I have thought about is to let the driver know when I am boarding the bus, so that he can vouch for me in case the inspectors do not believe me. However, what about the metro, where one can not talk to the driver?