According to the bishop at my church, there are four types of knowledge.

1 – When you know something, but you act like you don’t know it.
2 – When you don’t know something, but you act like you know it.
3 – When you don’t know something, and you know that you don’t know it.
4 – When you don’t know something, and you don’t know that you don’t know it.

He says the 4th one is the worst, and I agree with him.