UPDATE: Since this piece was written, the grace period has sadly been reduced to two minutes.
For pay as you go passengers in London, the unlimited 'Hopper' bus fare, introduced in January 2018 was a boon.
It means that, within the space of an hour, you can travel on unlimited buses and trams for a flat fee of £1.50.
But the thing about that 60 minutes... it's actually 70 minutes. That's right, there's a 10-minute grace period, which means that you might be able to squeeze in another cheeky bus ride for your £1.50.
Let's say you go shopping in Marble Arch's Primark, but don't find everything you need, so catch the 25-minute bus to Tottenham Court Road's Primark — and spend another 40 minutes shopping like mad there. You've actually still got enough time to hop a 'free' bus home, loaded up with your stash, rather than forking out on another tube journey. (Or to bus it back to Marble Arch's Primark if you've already decided you want to return that geographically confused tea towel.)
It's just an example. Primark is obviously awful. But the possibilities are there.
When Geoff Marshall trialled the unlimited Hopper when it launched in 2018, he managed to catch 25 buses within the hour. And although he did briefly reference the grace period, he didn't take advantage. Reckon you could have clocked up another three buses in that time, Geoff.