I recently interviewed for a position where I had to say I don't know, but I'll figure it out. Its was a weird situation because I knew the answer, but nerves wrecked my mind, and could not think straight.
Did not get the job. Even made a thread about it in Ask::HN.
But not knowing something is a great motivator for me. Something that I think employers should value.
Of the 100 or so interviews I have done over the past year and a bit, only two or three have had cases where the interviewee did not need to admit they did not know. If my goal is to understand the breadth and depth of knowledge, then one can only really have a clue about that when one starts hitting the edges.
The grace/confidence in admitting they don't know, the willingness to guess and explain how they came up to it (and continuing/iterating as I give more information), and how well someone integrates something new (ie, something I have just told them) later in the interview are all major considerations in my interview feedback and hire/no-hire decision.
Did not get the job. Even made a thread about it in Ask::HN.
But not knowing something is a great motivator for me. Something that I think employers should value.