If it is a scientist and the belief is in their field, then it's a good thing. Let Linus Pauling reach for crazy Vitamin C, or physicists believe in crazy E8 or whatever.

But yeah, outside of that it happens all over the place.

Most people are not trying to carefully cultivate the truth in the gardens of their mind. They have competing interests, especially group-membership interests which evolved for very important reasons (not finding yourself dead because people near you thought you were a traitor).