We used this sort of trick ages ago. I think we had four questions on our paper. And we simply wanted to compare programmers.
But the glaring incompetence it threw up was absolutely amazing! We made a point of saying "there isn't enough time to do the test justice, we just want to see if you've got the basics", but the number of people who took great trouble over question 1 and never got to the other three, or the people who completely misunderstood all the questions, or ...
The real corker was ten lines of code we lifted from one of our programs. The question was "what does this code do, could you do better?". Someone had effectively rewritten the int function. I think only ONE person said "hey, this looks like the "int" function, I could do it in one line". Everyone else simply started rewriting it without determining what it did first ...