My contactless card can occasionally fail to work (even on payments of under £30), probably due to fraud avoidance (it reports "unauthorised"). If this happens in a shop, no problem -- I can just put it in the card reader and enter the PIN. If this happens on a bus, there's a problem: the buses don't have card readers (or at least the driver didn't know about it).
In this case, I asked the driver if he'd let me on anyway, and he did. I then unlocked the card by just paying for something in Leeds and entering the PIN -- I assume an ATM would also work. It hasn't happened since, but it was slightly embarrassing.