Note 7 at Jn. 6:42: These Jews assumed that Jesus was the son of Joseph. Jesus never said that. It was their own supposition. If you accept their supposition as true, then their conclusion would also be true. Their reasoning wasn't flawed but the "facts" on which their reasoning was based were wrong.

Likewise today, we have accepted many religious prejudices as being true which are not and that leads us to wrong interpretations of God's Word. We have to make a conscious effort to make sure that the foundations of our faith are anchored firm in God's Word and not on the traditions of men (Mt. 7:24-27).