What I have found to be interesting is that the more I learn about a technical topic, the less confident I become. Because I am now more aware of how much I don't know, or about how a small detail can make a big difference.
I used to think I was pretty knowledgeable about web security. The more I learn about it, and about how easy it is to get it wrong, or overlook something important, the less confident I feel in my ability to implement a secure web app.
You know, the person who knows nothing about a topic, but will talk for hours on it?
There should never be shame in admitting "I don't know" and then following it up with, "but if faced with that I would..."