Don't know! oh my god, you do not know this? or he does not know even this? -- guess from where it is coming from. Most of the system administrator expect you to know everything on this world what they know - or Most of the consultant (being a consultant, I have learned not to say this) but IF in this world - some one is hiring you to do something - that means, one they do not have expertise to do that or they have some expertise but they are not able to do something to get it working; second - the most important thing - thats why you have the job. If some one will know how to fix network adaptor, apply permissions and play with IPTables on Linux, then why would they hire a Linux Administrator? OR everyone in this world will know LISP, how to create spreadsheets (useful ones) in Excel or Windows/Firewall Adminstration?
Dude, get a life, if some one is asking something to you, instead of expressing your "oh my god, you even do not know this" expression or discussing with collegues "dude, do you know that client dude does not know this thing" expressiong, you better help him/her out, and help him out the way he/she can understand it and better not ask you again. But if you like rubbing the coal on the wall, again and again! then please please go ahead and just do it on your own.
But humiliating something, or showing that you know the stuff and they do not? or keep pointing out mistakes and discussing lack of knowledge of someone, is specially not good.
Just think, how you do not know! instead of straightening your neck with what you know! because there is always a chance that some one might just tell you - "don't know"
One of my friend and myself discussed this yesterday while taking a walk! and I think its good they do not know because thats why I have the job! else I would have not had the job....