  We report results from an 18-day ROSAT HRI monitoring campaign on the
ultrasoft Narrow-Line Seyfert 1 (NLS1) class quasar PHL 1092. This luminous,
radio-quiet quasar showed strong X-ray variability in a short ROSAT PSPC
observation, and ROSAT HRI monitoring of the similar object IRAS 13224-3809
revealed extreme variability on intermediate timescales. We wanted to determine
whether remarkable X-ray variability persistently occurs in PHL 1092, and we
also wanted to search for outstanding variability events that constrain
emission processes. Given the large luminosity of PHL 1092 (about 5x10^{45}
erg/s in the HRI band), we detect extremely rapid and large-amplitude X-ray
variability throughout our monitoring. The maximum observed variability
amplitude is a factor of about 14, and in the most rapid variability event the
HRI count rate increases by a factor of about 3.8 in a rest-frame time interval
of < 3580 s. The most rapid event has a rate change of luminosity of >
1.3x10^{42} erg/s^2, making it the most extreme such event we are aware of from
a radio-quiet quasar. Standard `radiative efficiency limit' arguments imply a
radiative efficiency larger than can be achieved by accretion onto a Kerr black
hole rotating at the maximum plausible rate, although we point out that such
arguments depend upon the geometry of initial radiation release. Relativistic
motions of the X-ray source are probably causing the radiative efficiency limit
to break down; such relativistic motions have also been inferred in the similar
NLS1-class quasar PKS 0558--504.
