I had about a 45 minute phone interview with a senior engineer. He asked the standard questions about my background, experience with different technologies, what I was looking for in the new position, etc. Then he asked me a single question about a problem that he was currently working on and how I would handle it. At a high level I think that's a good interview strategy. It definitely required me to think on my feet, and I like that. However, I could tell that he literally picked this problem off of the top of his head. Without preparation, his problem description was very fragmented and it took a lot of questions for me to even understand the question. I'm fine with on the spot interviews like that, but if you're going to ask a complex technical question then you should have the problem well defined and be able to explain it clearly.