I haven't had the opportunity yet but I really want to take a piece of code from our system, de-optimize it a bit and add some bugs, then give users a unit test around it.
Most of what we do as engineers is trying to read code to figure out what it's doing, how to fix it and how to enhance it. Going through real code with a candidate and assessing their ability to understand brand new code (and what questions they ask) seems like it would give good insight to how they would perform on the job.
In one of the best interviews I had, the interviewer had printed off an actual (undocumented) class in one of their projects (I got the job, so I confirmed it later), then asked me to tell him what it did, and if there was anything I would do to improve it. I think he might have added a couple of logic or syntax issues as well, but I can't quite remember anymore.
I've been on dozens of interviews, and that's the only time I've ever encountered that, but I think it does a really good job representing the job, as that's invariably what I'm going to have to do when you hire me, is sit down and familiarize myself with the code base.
He also asked me a handful of questions, said "Okay I know you know enough to do the job, let's see how much you really know," asked me a bunch of really low level stuff, and proceeded to teach me concepts when I told him I didn't know certain things. I walked out of that interview knowing more than when I walked in, something else that has never happened in all the other interviews I've had (except maybe I learned a new term that I never heard before that I was apparently supposed to regurgitate back because that's what was written down as the answer on HR's answer sheet).
Most of what we do as engineers is trying to read code to figure out what it's doing, how to fix it and how to enhance it. Going through real code with a candidate and assessing their ability to understand brand new code (and what questions they ask) seems like it would give good insight to how they would perform on the job.