Never ever ever be afraid of saying that you dont know something. There is nothing worse that a self proclaimed know it all who pretends to know everything, and tries to bluff thier way through something they dont know, giving incorrect information more often than not.

In an interview or when dealing with a client, I dont know shows you dont know everything. who could possibly know everything? In an interview situation, follow up on any questions you didnt know, ask questions, show them that you are genuinely interested and willing to learn anything that you dont know.

On a client site follow up "i dont know the answer" with "but I will investigate and get back to you by xx:xx time tomorrow". I have often been asked something i didnt know, and as long as i am willing to find out, and set a clients expectations they are always happy with the answer. just make sure you follow up!