  We discuss a systematic uncertainty in time-resolved optical conductivity
measurements that becomes important at high pump intensities. We show that
common optical nonlinearities can distort the photoconductivity depth profile,
and by extension distort the photoconductivity spectrum. We show evidence that
this distortion is present in existing measurements on K$_{3}$C$_{60}$, and
describe how it may create the appearance of photoinduced superconductivity
where none exists. Similar errors may emerge in other pump-probe spectroscopy
measurements, and we discuss how to correct for them.
