While looking for some "pretty picture" for the DP0.1 data, Jeffrey Carlin noticed some "green, point-like sources that are slightly offset from galaxies in this DC2 (gri-bands) images". In a 'lengthy" discussion on this slack thread (108 replies and counting!), James Chiang narrowed it down to a bad WCS for detector 64 of the r-band visit 908967 (whereas surrounding fits looked ok).
It was noted that, upon a new processing of the exposure, the "badness" of this fit was revealed in the scatter output from the astrometry task to the logs:
whereas, e.g., the neighboring detector 65 has a more "typical" scatter of:
It was also noted that this bad fit was actually flagged by running checkCchAstrometry.py during the processing, but apparently these flagged detectors were not omitted from from the coadd assembly. Johann Cohen-Tanugi has provided a full list of such bad fits from the DP0.1 run, attached here.
Look into what is going wrong with these WCS fits, starting with a deeper dive on detector 64 of visit 908967.