So we went to Naples during one of these weekend trips, and we went to a restaurant our professor said had some of the best Napolitan pizza around. We had reservations, so all fifty-something of us were set up at tables fairly quickly, and got our food without too much of a wait.
Cultural FYI: normally, at Italian restaurants, the gratuity is included in the bill, so there's no need to tip more than a euro or two for good service. When my table was finished, though, our waiter came up to us and said, "Tip not included. You need to tip me." We were tired and didn't really question it; when the waiter came back, he hovered over our table as we figured out the bill, saying, "You're going to leave money for me, yes? You'll leave money on the table for me?" Ultimately, I think we gave him about eight euro for tip.
When we were walking back to the bus, all of our classmates said that similar behavior had happened at their tables, where the waiter had lingered by them until they'd left money out for him. When word got around to the professor who'd recommended the place, she looked surprised before telling us that the gratuity had been included in the bill all along. Apparently, the waiters had hit up every table (except for the professors') for extra money, and had gotten between 7-10 euro from each one.
Ultimately, they probably got about 45 extra euro out of us. From then on, we were basically told to not give out big tips anymore, though thankfully no one else on the trip tried to squeeze us for more.