Evidence of systematics in Gaia DR2 PMs. Top left panel: distribution of Gaia DR2 PMs for stars in common between Gaia DR2 and the HST NGC 7078 PM catalog of Bellini et al. (2014). Top right panel: same as the top left panel but for HST PMs. Bottom left panel: same as the top panels but for differences in the Gaia DR2 and HST PMs. Obvious outliers are highlighted in red for Figure 11. The dotted line illustrates the orientation of the Galactic plane. Bottom right panel: positions of Gaia DR2 sources with astrometric_n_good_obs_al below 110, which imprint the scanning law of Gaia. The orange polygon marks the field of view of the HST catalog.