Unauthorized photos in my private photostream

Today someone posted 3 photos to one of my photostreams as me. This stream is shared with members of my family, but I did not post the photos and none of the family members did either. My settings are set to only subscribers can post, public website is off and notifications are on. This seems like some sort of iCloud breach or permission issue that should not be allowed. Has anyone else seen unknown photos appear in their private photostreams?