  Nonlinear terms in the equations of motion can induce secularly growing loop
corrections to correlation functions. Recently such corrections were shown to
affect the particle production by a nonuniformly moving ideal mirror. We extend
this conclusion to the cases of ideal vibrating cavity and single
semitransparent mirror. These models provide natural IR and UV scales and allow
a more accurate study of the loop behavior. In both cases we confirm that
two-loop correction to the Keldysh propagator quadratically grows with time.
This growth indicates a breakdown of the semiclassical approximation and
emphasizes that bulk nonlinearities in the dynamical Casimir effect cannot be
neglected for large evolution times.
