Very often in Calculus, there are some conventions about ‘subtle’ matters. According to the particular definition of some concepts, a one-valued real function can be named “discontinuous” or “not defined” depending on the point considered inside the domain or in the domain, including boundary, in this latter case. In Calculus, this subtlety on discontinuity it is just a matter of convention, in Physics, this subtle difference gives substantial consequences. In this paper, the physical problem linked to the Calculus matter is obviously intended as an ideal problem solving situation.