1) You're looking for someone with deep knowledge of that subject
2) You don't treat it as an automatic fail when they get it wrong
If I'm interviewing for a senior role, and I ask them a few super obscure questions about the language/framework/platform I need them to be an expert on, that serves as a litmus test for their overall depth of knowledge. Those facts individually may not be required for the job, but they give a read on how deep the person's knowledge goes in general.
But, as you pointed out, I wouldn't really even put closures in that category. For JS it would be something more like, "What does '{} + 12' evaluate to?"
