There should be a sliding scale depending on how many minutes late you are, I reckon...
Better still, they should abolish Pay & Display in favour of Pay on Exit. I've hated Pay & Display for a number of reasons ever since it was introduced. You often end up buying more time than you need 'just in case', or conversely you end up rushing back to your car when you might otherwise have decided to stick around a little longer.
If you're staying somewhere all day, you're basically sticking an ad in the window saying "by all means steal my car. By the time I'm back and call the police, you could be in another country."
The wife once got fined in a Pay & Display car park because her ticket fell on the floor. She contested it and managed to get off with the charge, but the faffing around and stress that it caused were totally unwarranted considering that she'd done nothing wrong. We were still out of pocket for the cost of sending the ticket via recorded delivery.
In all honesty, if I'm ever in this situation, and the machine isn't one of those that takes your registration number, I will hang around waiting for someone to come back to their car, offer them a few quid for their valid ticket and then swear black is white that I had a valid ticket in my car, and that I can produce the ticket on request to prove it.
Maybe that should read 'in all dishonesty'
If you want to hear about stupidity, you need to see the 'free after 3' scheme that our local council is operating. As you'd imagine, you can park free after 3pm, but (and here's the good bit) you still need to get a (free) ticket from the pay & display machine. I didn't realise this when they first introduced it, and just started dumping the car there without paying after 3pm. It turns out that not only do you have to take a ticket, but the wardens are still out there issuing fines for cars that aren't displaying a valid ticket. You have to wonder whether it's a cynical ploy to confuse motorists in the hope of raising more in fines than they would in pay & display tickets.