Thats exactly what happened for me in my school trip!
Me and someone friends went to this one cafe on the beach and each of us ordered something,they ordered two big milkshakes and I ordered a glass of martini.After we were done she brought us the check and suddenly they decided to order one more milkshake for them (there was three of us) and when she brought it to them they were like "No no,we don't have enough money for that,you can take it back".
But while she was taking it back,they just left the money and ran.I was so embarrassed in front of that poor girl,eventually I left her a 50% tip for the service and the inconvenience.
And thats the last time I got somewhere with those cheapass idiots >__>