I only see people indicating backwards on roundabouts (indicating to exit when not exiting; indicating to stay on the roundabout when they're exiting). I presume they got bad instructions.

I can imagine that roundabouts might seem confusing. I have no idea how someone can get indicators wrong on other types of intersections

I think part of that is caused by stupid turn signal designs that don't stay in place, so to cancel a signal you have to push it in the opposite direction. And people can't figure out how to get them to work properly.