Too Much Self-Confidence
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.
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.