Common mistake made by either techie neophytes or Jr. I.T. people is that because you are in I.T. you know something about everything in I.T. It is no different, actually it is different in that I.T. is a wider subject matter, than saying because you're in science you know about planets or delivering babies or brain surgery. I.T. is a very broad term.
I have been in the industry for over 25 years and the amount of knowledge and expertise I have is minuscule compared to I.T. knowledge as a whole. I have interviewed, trained and mentored many I.T. people as well as consulted for I.T. professionals and I can always tell a seasoned I.T. person vs. a Jr. I.T. person by how much they think they know on the subject matter. A Jr. I.T person, with a bit of experience under their belt and a lot of confidence, will believe wholeheartedly that they know quite a lot, where as the seasoned I.T. person will admittedly say they now very little.