It happened to my two French friends, and it was a pity for me cause it was a restaurant which I recommended. I used to dine there many times in Krakow, and they never scammed me, yet they did it to them. They just asked for tap water and were given a jug, but afterwards were forced to pay for it 15 zlotys (around 3,5 euro)! Just for a jug of tap water! I was really ashamed.