  We report on the discovery of a dramatic X-ray spectral variability event
observed in a $z\sim 1$ broad line type-1 QSO. The XMM-Newton spectrum from the
year 2000 is characterized by an unobscured power-law spectrum with photon
index of $\Gamma\sim 2$, a column density of $N_{\mathrm{H}}\sim 5\times
10^{20}\,\mathrm{cm^{-2}}$, and no prominent reflection component. Five years
later, Chandra captured the source in a heavily-obscured, reflection-dominated
state. The observed X-ray spectral variability could be caused by a
Compton-thick cloud with $N_{\mathrm{H}}\sim 2\times 10^{24}\,\mathrm{cm^{-2}}$
eclipsing the direct emission of the hot corona, implying an extreme
$N_{\mathrm{H}}$ variation never before observed in a type-1 QSO. An
alternative scenario is a corona that switched off in between the observations.
In addition, both explanations require a significant change of the X-ray
luminosity prior to the obscuration or fading of the corona and/or a change of
the relative geometry of the source/reflector system. Dramatic X-ray spectral
variability of this kind could be quite common in type-1 QSOs, considering the
relatively few datasets in which such an event could have been identified. Our
analysis implies that there may be a population of type-1 QSOs which are
Compton-thick in the X-rays when observed at any given time.
