What are we supposed to do when someone totally misjudges us and actually believes what they are saying is true when they are totally off base?

Some people are very bad at judging someone's character and tells them what they think about them.
How can you defend yourself when they are so wrong about you and they're just bad at judging someone's character?
What are we supposed to do when someone totally misjudges us and actually believes what they are saying is true when they are totally off base?
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