[gonehome]

I think it has gotten more sophisticated to detect cropped images and small changes now: https://inhope.org/EN/articles/what-is-image-hashing

The example is somewhat contrived.

If a 'friend' takes your phone and has access to it and then uses it to take images of CSAM similar enough to the original image that it triggers the hash match and does this enough times to go over Apple's threshold to flag the account after these images are uploaded to icloud without the original phone owner noticing then yes it might cause a match.

At that point the match is probably a good thing (and not really a false positive anyway) - since it may lead back to the friend (that has the illegal material).

[zbobet2012]

Or you know, anyone who wants to plant material on a device and has physical access. Say a disgruntled employee before leaving, or ex, or criminal or...

Or anyone who can just text you since imessage backs up to icloud automatically...

[jeromegv]

While iMessage backups to iCloud, this measure is only for photos stored in the iCloud Photo Library. So sending a text with the photo is not enough.