Equinox (celestial coordinates)
The Hipparcos Catalog uses the International Celestial Reference System (ICRS) coordinate system (which is almost completely equinox J2000.0) but uses an epoch of J1991.25.  For objects with a significant proper motion, assuming that the epoch is J2000.0 leads to a large position error.  Assuming that the equinox is J1991.25 leads to a large error for nearly all objects.