  We present a review of the topics of X-ray stellar tidal disruption events
(TDEs) and changing-look active galactic nuclei (AGN). Stars approaching a
supermassive black hole (SMBH) can be tidally disrupted and accreted. TDEs were
first discovered in the X-ray regime and appear as luminous, giant-amplitude
flares from inactive galaxies. The early X-ray observations with ROSAT also
established the extreme X-ray spectral softness of these events with
temperatures of order 50-100 eV that continues to be seen in the majority of
more recently identified events. While the majority of X-ray TDEs has been
identified from {\it inactive} galaxies and some showed the highest amplitudes
of variability recorded from galaxy cores (amplitudes exceeding factors of
1000--6000), a small fraction of {\it active} galactic nuclei (AGN) has been
found to be highly variable as well. In AGN, this so-called changing-look
phenomenon often comes with a strong change in the optical broad emission
lines, leading to Seyfert-type changes between class 1 and class 2. These two
forms of activity represent the extremes of variability among active and
quiescent galaxies, and have opened up a new window on understanding accretion
physics under extreme conditions. Finally, we introduce the term ``frozen-look
AGN'' to describe systems that show constant line emission despite
strong/dramatic changes in the observed ionizing continuum. These systems are
best explained by strong changes of absorption along our line-of-sight.
