I found this Mark Twain’s quote at Herding Cats blog:

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.

I find myself very often arguing with people who are so sure about the things they say and they never agree with me even when I bring hard proofs in front of their eyes.

Self-confidence is good. Shy people never succeed. But sometimes, if you are too confident in your skills or knowledge and you don’t bother to check the facts, you may really get into trouble.

Mark Twain said it perfectly well.