So I pull out trusty old Ruby + RubyMine, and the question (I don't remember what it was specifically, but it was some sort of array manipulation type of thing) was trivially solvable with some Ruby STD lib method. Apparently he wasn't satisfied with the answer despite his prior instructions, and me not knowing what the actual method code is was reason enough to disqualify me from the job, which I found baffling.
I got into a bit of a heated discussion that, if I were solving a problem at work, there's a 99% chance I'd just reach for this method that Ruby itself comes with, because why the fuck would I try be a smartass and reinvent stuff here? And if for whatever insane reason I did indeed need a bespoke solution, I'd just poke around the source code and extrapolate from there, which he still didn't consider a good answer.
These interviewers drive me insane sometimes...