  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 (less than one percent 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.
