This one is quite a big quality of life improvement IMO. It used to be that I sometimes left my browser accidentally open on facebook, and people would get offended because I 'read' their messages and didn't respond.
Maybe telling your friends that you didn't actually see their message and that instead the algorithm FB uses to record my mouse activity sometimes reports that I have read messages that I have not. This sounds dangerous and could be harmful to a person's credibility if another with bad intentions acts.