I do this, but with the goal to find a specific thing they don’t know in an area they do know. Then I want to see them work out what a reasonably possible answer would be
I'm 100% fine with that. I had one interview where we ended up talking about the best data structures for a visual editor to store text files in-memory. It wasn't related to my day job at all, but I walked away feeling like I'd learned something, and the interviewer got to watch me reason my way through unknown territory to see how I handle such things. That was fun. I have no idea if I got the "right" answer or not, but it was at least defensible, and I stumbled across some ideas that he seemed to find unexpected and interesting. I ended up getting the job.
What made it enjoyable was me knowing that I wasn't expected to know the gory details of how text editor internals work.
What made it enjoyable was me knowing that I wasn't expected to know the gory details of how text editor internals work.