One of my best interview experiences was exactly that. The interviewer handed me a printed out class and said "tell me what this does, any mistakes you see, and any little improvements that you think could be made."

I got the job and later confirmed it was a class that was actually in the software I would be working on, not some toy class he came up with.

He also asked me a handful of technical questions and had me go over some source code I brought in, but that was about it. At one point he even said, "I know enough to know you can do the job, but now I'm curious what you really know." And asked me some no pressure really low level questions. When I said I didn't know the answer to something he'd spend a couple of minutes explaining the concept to me.

I walked out of that interview having learned a few new things about my field. A couple of them have stuck with me over a decade later.

And the guy was a grizzled veteran who was the lead on some massive mainstream projects I pretty much guarantee you've heard of. He's probably the most knowledgeable programmer I've ever met.

I've never had anyone try going over actual with me since, in probably a couple dozen interviews, and I don't understand why. It seemed very effective.