Today it didn't prompt at all - instead it quickly flashed up "Authorised" on the screen before saying "Please remove card"... and then printing out a space for me to sign. We all thought that was rather odd, but I signed it anyway (with a pencil, as the staff didn't have a pen!) and left it at that. Then a little while later I checked the receipt out of curiosity and spotted that it said "Refund". And that instead of "Please debit my card", it said "Please credit my card". It appears that the waitress had somehow put through my bill as a refund by mistake!
Well, myself and the GNU went back to try and work out what had happened and if necessary actually pay for lunch. The waitress was as surprised as we were by the whole thing, but in the end decided to undo the refund with an equal charge, then repeat that for the actual payment. Except on the second payment, she accidentally put it through as a refund again! This time she caught it and managed to cancel it, and then finally correctly entered the payment for lunch. I now have a collection of receipts for various states of credit and debit transactions, and probably have a rather confusing bank statement to look forwards to at the end of the month.