You don't say what camera you were using but since you're posting in the DSLR forum I assume that you were using a DSLR. Okay, so when using my Sony NEX-5 APS-C cameras I've seen something similar now and then and I think it is probably a bug in the camera's firmware or a glitch with the sensor. It doesn't happen that often but every once in a while a frame will have a very different color balance (for lack of a better term). I suppose it could even be a glitch in the way the Bayer pattern is being recorded in the frame (throwing off the debayering).
I've also seen frame instability in one of my dedicated astro cameras (an ASI174MM) and in those cases I think it is a bug in the driver. In this case it's not like a change in the color balance but a gross change in the amount of amp glow, such that the frames are almost unusable. The common fix in this case is to do a power cycle on the camera. Fortunately, this latter issue doesn't come and go so I can usually catch it before I start my imaging for the night.
In both cases I think it is caused by a random bug somewhere in the firmware/driver or a simple glitch in the sensor itself. My solution, since it happens so infrequently is to ignore the issue. This is one reason why it's always a good idea to do a blink inspection of your files (both lights and darks and bias if you use the latter). Either that or run a quality selection tool like PixInsight's SubframeSelector. Practically speaking, however, if you don't notice any problems in your finished master then it may not be something that you should even check for (assuming that it happens very infrequently -- like a few frames once in a while on different nights).
Edited by james7ca, 18 June 2021 - 10:44 PM.