I have had this go both ways but much more often in the overcharging category.

A few weeks ago in a restaurant in Cagnes sur Mer, France the bill arrived with the appetizers not included and only one of two bottles of wine billed. I brought it to the attention of the person brining the credit card machine and she said "Mr. jbcarioca, you never have two bottles of wine and we were so slow that it should be free anyway" It is a restaurant we often visit but that was a magnificent gesture anyway. The restaurant
http://www.palais-dasie.com/ is a very good Chinese/fusion in the French style. When they treated me so well I had to give them a plug, did I not?