I must admit I would prefer someone to say "I don't know, but I'll try and find out" rather than the first thing that comes into their head. I have moved jobs a bit in my life and the inrterviewers were always pleased when I said "I don't know" to some technical questions. Rather than make s**t up they would much prefer an honest answer.
I am the same on the forums, if I know the answer (usualy because I have been there and done that) to a technical question, I will not offer an answer if I am not 100% sure. Or, if I do offer an answer I make clear that I am not certain.

Ivan