  We discuss the properties of a small sample of Seyfert 2 galaxies whose X-ray
spectrum changed appearance on time scales of years, becoming
reflection-dominated from Compton-thin, or viceversa. A reflection--dominated
spectrum is usually taken as evidence of Compton-thick absorption, but we
instead argue that such a spectrum is due to a temporary switching--off of the
nuclear radiation. The observations discussed here may help explaining
mismatches between optical and X-ray classifications, and provide new strong
and direct evidence of the presence of more than one cold circumnuclear region
in Seyfert 2 galaxies.
