A photographer could accidentally upload or AT SOME POINT labelled his/her photos in the free domain, but ALSO upload with a license to a paid photo stock site.

A photographer could accidentally upload or AT SOME POINT labelled his/her photos in the free domain, but ALSO upload with a license to a paid photo stock site. This is usually the root cause of most issues. Another possible reason is the photographer's harddrive got hacked, camera stolen etc, and the photos in full resolution was uploaded maliciously to free CC0 websites. Both scenarios are hard to trace and hard to attribute when an infringement happens, because it involves too many party to do diligence and collect evidences. Most likely the party that uses the photo will have to pay. Canon and Nikon still provides no on system encryption although photographers have been asked for it for years, so basically if someone stole your camera, they also owns the copyright of photos from those RAW files.