Let's not forget that Apple want us to trust them to review our image files before they call the police, when their client-side content scanner detects too many

Let's not forget that Apple want us to trust them to review our image files before they call the police, when their client-side content scanner detects too many matches against an opaque police database of hashes. I don't think Apple have revealed the specifics of what process their human reviewers use (as a "security by obscurity" approach, and to prevent the bad press of admitting that they potentially subject low wage contractors to reviewing mentally damaging illegal images), but it's at least conceivable that an iPhone user could receive a series of images which look like (for example) bad scans of paper documents, but, when rendered on a reviewer's screen, look like images that require police attention.