A simple relativistic paradox about electrostatic energy, Wolfgang Rindler and Jack Denur, Am. J. Phys. 56(9), September 1988
A charged parallel-plate vacuum capacitor moves uniformly through an inertial frame. Its field energy alone does not transform according to the familiar law "energy = g x rest energy." However, when the stresses in the support are taken into account, the entire system does satisfy this relation.