I don't think I ever saw an XCKD strip that wasn't obvious. But then I dropped following it a few years ago. Their attitude was much too irritating, and then they would let some kind of emabarrassing lapse every now and then. I think what drove me to drop at last was a strip where a character talked about the fastest way to cross a lawn. It was immediately obvious to me that the problem was isomorphic to the light refraction, and therefore the answer was to follow the Snell law. It could even be resolved analytically with only trigonometry if lawn's edge was straight. But apparently the author of the strip didn't realize how simple it was. Yet he writes strips with this hollier-than-thow and uber-geek attitude.