Interestingly, the International Prototype Kilogram (IPK) stored in France upon which the definition of the unit of mass is based on has been found to vary in mass compared to its national reference prototypes during the three verifications that has been performed in 1889, 1948 and 1989. This is of course troublesome since the mass of the prototype made of platinum and iridium by definition is one kilogram regardless whether it has lost or gained mass.
Countries around the globe has replicas of the original prototype used to calibrate national instruments. These replicas of the original prototype vary even more due to more common usage including scratches and contaminations. As an example, the American replica K4 has lost 41 µg compared to the international prototype kilogram.
The underlying problem is of course that the unit is based on a physical artifact. A similar artifact, a platinum-iridium bar with two marks on it, was the base for the definition of the metre up until 1960 when it was changed for a definition based on invariant physical constants (currently the speed of light). The kilogram is the last base unit with this type of definition and the International Committee for Weights and Measures voted in 2010 for the development of a proposal where the definition of the kilogram is based on the fundamental Planck constant. There are currently different ideas how to realize such a definition with high enough accuracy.