Could get into an interesting discusion about issues such as legal tender etc. For example if you put 30 of petrol in and tried to pay with 60 50p coins you could be refused as it would not be "legal tender". The individual coins would be ok but the quantity not.
Perhaps not quite the same as with the C&P cards.
Now, if you filled up with petrol and they wouldn't accept your card unless you told them the PIN, as referred to in another sub-thread, that would be even more interesting.