Don't make up an answer or, get flustered! Professionalism is expected even under circumstances that are tricky, and your reaction and response will speak louder than a phony answer, or the wrong answer.
My best advice, is to make sure you carefully listen to the question, if you don't know the answer, take a short breath and gather your thoughts. Then respond in a professional manner, without being nervous: "I am sorry, but I have never heard of this specific form until just now. I will make sure to research it as soon as I get home."
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Here is an AWESOME write-up on "The Best Answers to Tough Interview Questions" by Carnegie Mellon Career Center:
http://www.studentaffairs.cmu.edu/career/cit/new/jobSearch/InterviewQuestions.pdf