This demonstrates why an app ticket must be bought before boarding the train in terms of revenue protection policy. Although in your instance it was a genuine mistake, if you had been a freeloader then you could have bought that ticket and then walked straight out, having come from far afield. And if you are chancing it by boarding without a ticket then if you see a revenue inspector and immediately buy a ticket, everything will seem in order. But you're effectively getting a free chance to evade paying a fare, with no comeback if you're about to be caught. So a fare-dodgers dream.