I used to work at a company that used XSLT. They know that it's an obscure language that probably none of the potential candidates have ever worked with. But it's easy enough to learn the basics in an hour or two.
So the entry test was to strip some tags from an XML file. You had a day or two (maybe more) to do it. My solution wasn't ideal, I didn't use several of the shortcuts available in the language. But at least it did what it was supposed to.
A few weeks after I had started working there my boss came up to me, visibly frustrated and asked me whether the test was too hard. Thinking back on my problems I replied that maybe having the desired output ready so that you could test your own solution against it might be nice. But my boss's problem was that none of the last 5 candidates could even send in a solution that would run.
You had so much time, and running an XSLT script is really easy and takes no time at all. And for some inane reason these people couldn't even manage to test their code and still decided to send it in.
And I thought I was an idiot when I didn't know if it was spelled grey or gray in CSS during the in-person interview.