The completeness of USNO-B detections of high proper motion ({mu}>180mas/yr) stars is tested, and the accuracy of its measurements is compared to the revised New Luyten Two-Tenths catalog of Salim & Gould (2002ApJ...575L..83S). For 14.5<V<18.5, only 6% of such stars are missing from USNO-B (Cat. <I/284>), while another 3% have large errors, mostly too large to be useful. Including both classes, incompleteness is 9%. These fractions rise toward both brighter and fainter magnitudes. Incompleteness rises with proper motion to 30% at {mu}=1"/yr. It also rises to 35% at the Galactic plane, although this is only determined for relatively bright stars V<~14. For binaries, incompleteness rises from 9% at separations of 30" to 47% at 10". The proper-motion errors reported internally by USNO-B are generally correct. However, there is a value of {sigma}{mu}~4mas/yr below which the reported errors should not be taken at face value. The small number of stars with relatively large reported errors ({sigma}{mu}>~20mas/yr) may actually have still larger errors than tabulated.