T.R.O.L.I. You're missing the point slightly IMO - if you go to the bar with a £10 note and find it isn't enough, then you can get some more money out of your wallet. If you go to the bar and only find out after you;ve ordered that you've only got £10 on your card for a round of £12 then you're screwed (unless one of your mates is nearby with a card with some money on it).
No, I don't think he missed your point at all - but you have!
The scenario you post is equivalent to queuing up for £12 of purchase with only £10 in your wallet. You can't just wave a wand and find an extra £10 in there. You'd need a mate with some money in his wallet.
If you know you are going to buy £12 of stuff you need to check that you've got £12 (or more ) on the card.
Ergo, getting to front of the queue and finding that you only have £10 on the card is NO dfferent to finding you have only £10 in your wallet. In both cases you have made a mistake and will be unable to pay for your purchases.