My clients at Sparkbox pay me a lot of money to provide knowledge and expertise for their projects. In this dynamic, it's easy to feel pressure to sound smart. If you've done any consulting, you know what I mean.
But trying to sound smart isn't smart, and I've learned to embrace telling people when I don't know something.
Confessing a lack of knowledge isn't a weakness. It's an asset. Every time you say "I don't know" in a work environment you give yourself credibility. People hear "I don't know" and think "Ok cool. Their recommendations are legit, because if they don't know, they'll say so."
Credibility is more valuable than looking like you know some obscure bit of knowledge. Your clients will forget about the detail they were asking about, but they won't forget that they can trust you.
PS: "I don't know. Let me do some research on that and I'll get back to you" is a solid response in any situation. Especially when you follow through consistently and well.