Personally, I think it really comes down to what knowledge claim the person believes in. If this claim does not affect others, and only involves the person, then it most likely will not be harmful in any way. On the other hand, if it does influence the society and other people, then that may cause harm and the person should probably stop believing it.
If the knowledge claim simply involves a person without it having any form of impact on others, I think it is perfectly fine to believe in it even if it is false. Many people tend to believe in some kind of lie, for instance that their country’s food is superior to any other country’s food, or that they are better at doing a particular activity than everyone else. These small lies don’t really have much of an effect on others, and therefore will not cause harm.
However, there are false knowledge claims that can do harm towards the society and other people involved in it. This tends to happen quite frequently in schools when rumours are being spread. Most of the time, these rumours are false and yet people still choose to acknowledge these claims and believe that it is true. Obviously, the people involved in this rumour will feel upset since it is not the truth, but the other students continue to disregard the fact that it is a false claim. In this case, believing in false knowledge claims will do harm towards other people.