Our CCD has a number of bad pixels with lower than average sensitivity. They are a result of imperfect manufactoring, and most of them are collected in small groups. Their numbers and location are stable, and almost all of them are completely removed with even a mediocre flatfield.
But the most severe of the bad pixels, seem to have different strength in different master flatfields, and therefore they show up in diminished form in difference images between two master flatfields. The worst pixels may be 6-7% different from one master to the next, and therefore these pixels should be avoided. Luckily most of the bad pixels are close to the edge of the CCD. There is one semi-severe group in the central part of the CCD. I have seen examples of 1.7% variation in a couple of pixels from this group.
The figure is the twilight master flatfield in the IRCUT- filter from session JD 2455827, shown in power-scale to emphasize the small dark speck that are the bad pixels. The three most severe groups of bad pixels are circled in green, and the two second worst ones are circled in blue. These five groups of pixels should be avoided if possible – just to be on the safe side.