For objects with reasonable measurements in at least two bands, I plot the RA/dec of those objects in the 10 SN fields where mag_psf_g = 99.
Besides objects near chip boundaries (not covered so well with overlaps in the SN fields where the usual pointing is the same) and satellite trails,
there are some interesting features. In particular, in the SN E1 field, there is a dome shaped area with many objects with mag_psf_g. A search for nearby bright stars doesn't reveal any obvious possible candidates that might explain the observation.