Symptom: Artifact candidates are being clipped when there not sufficient epochs remaining to do so.
Cause: A crude N-Image is passed to filterArtifacts to compute the maximum number of epochs a candidate can appear in and be clipped. If there are < 3 epochs it's zero. (i.e nothing should be clipped). This N-Image is too crude.
The actual N-Image can be lower due to recorded chip defects (everything in the bad mask plane).
NOTE: This only affects coadds with small numbers of epochs