It`s the same kind of scenario as Argos had a few years back when they advertised TV`s on the website for �1.99 instead of �199. They sold hundreds before they realised, and only a few were honest enough to take them back. They argued to other customers who had bought one that because it was an obvious mistake whilst printing, the customers had to pay full price for the TV.
I think a lot of it is at your own discretion. Obviously, both the fuel and the TV`s were not at the correct price, so most people would just accept the full price and get on with it.
One final note, I am sure Retailers who advertise prices wether it is fuel prices or clothes, chocolates etc have a clause which says " Prices indicated may vary without notice" which means they can basically charge you what they like at the till, but again, discretion is observed.