This happened to me. I once interviewed at a place where they presented me with a language I had never seen. Very different concepts than your typical C/Algol flavored languages. They went over the basic concepts, with a handy reference sheet for me, then gave me a few tasks to complete in that language.
Towards the end, after completing the tasks, I congratulated them for inventing a ficticious language in order to test candidates on a more equal footing. Cute, clever, creative, even though it's probably far-removed from the real world...
Their response? Oh this is not a ficiticious language, this is what we use daily. It's our proprietary language and we're quite proud of it.
Me: ...
(I ended up getting the job and learned a lot in the domain of wheel reinvention.)