  We examine the regime of validity of $N$-point spectra predictions of single
field inflation models that invoke transient periods of non-adiabatic
evolution. Such models generate oscillatory features in these spectra spanning
frequencies up to the inverse time scale of the transient feature. To avoid
strong coupling of fluctuations in these theories this scale must be at least
$\sim 10^{-2}/c_s$ of the Hubble time during inflation, where $c_s$ is the
inflaton sound speed. We show that, in such models, the signal-to-noise ratio
of the bispectrum is bounded from above by that of the power spectrum, implying
that searches for features due to non-adiabatic evolution are best focussed
first on the latter.
