  We give an assessment of the significance of various known effects which may
produce genuine fluctuations of star positions comparable to or larger than
Gaia's measurement noise, and which thus may limit the ultimately reachable
precision of the determination of parallaxes and proper motions. Stellar
granulation is found to be no problem except for a small number of cool
supergiants. It will be a serious problem for e.g. Mira star parallaxes. Star
spots are hard to assess quantitatively. They, too, may be problematic for
supergiants and very cool giants. Binarity is a major problem for the
astrometry of bright stars. It will significantly impair the kinematics of
stellar aggregates with small velocity dispersion. It will also produce a small
proportion of grossly wrong parallaxes. Gravitational microlensing will not be
a problem, although thousands of highly significant microlensing events will be
detected by Gaia. Gravitational macrolensing of quasars will add some noise to
their (otherwise ``zero'') proper motions.
