When to say "I don't know" in an interview

Conventional wisdom may tell you to never admit not knowing something.  Especially during the course of a job interview.  But I am here to tell you of three instances where it is actually a good idea to do just that.

1)  The first time it is okay to say "I don't know" is pretty obvious - it is when you really don't know the answer to a question.  Sound like common sense?  Of course it does.  But I can't tell you how many times I have had candidates lie, embellish, concoct, and dance around the answer to a question that they don't really have.  If you don't have experience in a particular area, and the employer is asking about that area, don't be afraid to tell the truth.  Instead point to being a quick learner or ask a follow up question regarding how critical the knowledge is to the daily job duties.

2)  Another opportune time to say "I don't know" is actually before an interview even takes place.  It is when an employer or a recruiter or a networking contact asks you if you are interested in even considering another job.   It is okay to be uncertain of whether or not you would leave your current employer.  It is also okay to consider a conversation when the outcome could be to stay put.  I see candidates far too often shut down the idea of any discussion about another opportunity.  While it could be seen as ultimate loyalty to your current employer, you will never know what else is available without exploration.  Or perhaps you will need someone to help with your search down the road and you'll want people to remember you fondly.

3)  I suspect this point will foster some debate, but saying "I don't know" to this interview question is far better than making something up that you don't plan on executing.  The question is "Where do you see yourself in five years?"  Most people will tell you to have an answer prepared for this question.  But I see a healthier dialogue coming from providing "I don't know" as your answer.  Clearly you will want to follow up with some additional details or questions on your end.  But unless you have received all kinds of assurance from the prospective employer that they can provide a good foundation for these five-year goals, you have some work to do to understand their culture and how many employees they have that celebrate their five-year anniversary with the company.  Until you know that, I don't know how you could honestly paint a picture of hitting that milestone.

Have you ever experienced another time in an interview where "I don't know" was the best answer?