Here's one I got awhile back. I'm not sure I did that well on it. Not because I didn't solve it. But because I'm more of an intuitive than an analytic, I don't think he liked my explanation.
You have a disk painted half black and half white, mounted at the end of a shaft so it spins like a platter.
At the 12 o'clock position, you have a sensor that you can test that returns true if it see's white, false if it see's black. You want to determine the direction the disk is spinning. How many more sensors do you need? Where do you put them? Why? How fast must you sample relative to RPM for your data to be valid?
I reckon tonytib knows this one cold - but having no mechanical actuator experience, I had to solve it on the spot from first principles.