Mark Twain: It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.

It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.
- Mark Twain (attributed)

If you know it "for sure," it probably isn't truth to everyone else. We are most sure about those things to which we have given the least thought. If you KNOW that something is true without having ever questioned it, take the opportunity to consider the opinions of others with an open mind.