I have interviewed couple candidates that were offended by the fact that I am asking hard questions that I know answers to.

This resulted in remarks like "I have never seen this question on the Internet" or "So what is the answer to this question anyway?" or "If I knew questions would be this hard I would not bother to come".

Somebody explain to people that it doesn't matter how hard the questions are but how the candidates compare. I am fully prepared that the candidates study questions that are available on the Internet, I want to see how they deal with something that requires a bit more than couple hours of effort and rote memorization.