they don't know which are which. some ideas they think are good are actually mistaken. some ideas they think are mistaken are actually good.
so then we can look at lots of a person's ideas and evaluations and ask: what if this one is mistaken? how might they find out? how might they fix it? if they're mistaken and they never find out, that means they won't fix it. is that a big deal?
often it is a big deal, and there's no serious, realistic efforts going into finding out what one is mistaken about.