  We report the detection of OVI 1031,1037 and NV 1238,1242 absorption in a
system of "mini-broad" absorption lines (mini-BALs) previously reported to have
variable CIV 1548,1550 in the quasar PG0935+417. The formation of these lines
in an extreme high-velocity quasar outflow (with v ~ -50000 km/s) is confirmed
by the line variability, broad smooth absorption profiles, and partial covering
of the background light source. HI and lower ionization metals are not clearly
present. The resolved OVI doublet indicates that these lines are moderately
saturated, with the absorber covering ~80% of the quasar continuum source
(C_f~0.8). We derive ionic column densities of order 1015 cm^(-2) in CIV and
several times larger in OVI, indicating an ionization parameter of log U >~
-0.5. Assuming solar abundances, we estimate a total column density of N(H) ~5
x 10^(19) cm^(-2). This outflow emerged sometime between 1982 and 1993. Our
examination of the CIV data from 1993 to 2007 shows that there is variable
complex absorption across a range of velocities from -45000 to -54000 km/s.
There is no clear evidence for acceleration or deceleration of the outflow gas.
Outflows are common in Active Galactic Nuclei (AGN), but extreme speeds such as
those reported here are extremely rare. It is not clear what properties of
PG0935+417 might produce this unusual outflow. In fact, PG0935+417 has
significantly less X-ray absorption than typical BAL quasars even though its
outflow has a degree of ionization typical of BALs at speeds that are 2-3 times
larger than most BALs. These results might present a challenge to theoretical
models that invoke strong radiative shielding in the X-rays/far-UV to moderate
the outflow ionization and thus enable its radiative acceleration to high
speeds.
