As a child I was always told not to drink after eating lots of fruits, esp. cherries, as it would supposedly cause me belly-ache. I never dared to try it and I see this to be observed quite strictly by most people around here, therefore I have no first hand experience. The reason given at Around the World Q/A seems almost the same as in Is drinking water during a meal bad for you?, which was determined to be wrong. What is the truth, is it harmful or not?
As a bonus, if it is proven to be a nonsense, it would be interesting for me to known if this is a habit only local (I live in Czech Republic, Central Europe), or if people around the world often think the same as well.