For example, the NCMEC database contains hashes for Nirvana's Nevermind cover. Completely innocent to possess within it's original and intended context.
I have not said whether I agree with this, because I do see problems when automating the process.
However, the precedent for it being used as a flag for law enforcement has already happened. Having a collection of similar imagery is considered CSAM - and that is likely correct. A collection is probably not innocent. But having one or two images may happen incidentally without your awareness of it.
As to who decides what constitutes significance? That is where you'll hit the most problems, and reasonable discussion of it will be quickly shut down with the same arguments used for automating a flagging system. The conversation requires nuance, but those currently calling for such systems aren't interested in a good faith discussion.