We discuss a universal bound on any excitation of heavy fields during inflation : the ratio of the heavy field ’ s energy density to the one driving inflation must be less than the maximally allowed relative amplitude of oscillations in the power-spectrum ( \rho _ { \mathrm { h } } / \rho _ { \mathrm { I } } \lesssim 0.01 according to PLANCK ) . This bound can be traced back to the sudden change of the equation of state parameter across the excitation event . We employ a sudden transition approximation at the perturbed level , which has been used before in different settings ; we check its validity by comparison to the full multi-field result in a concrete case study involving a sudden mass change of an inflaton .