The one I talked about before, but it is very obscure, even unknown on the whole World Wide Web.
We have two equal stars, A and B. Then we have a planet called X in the orbit around A, and a planet called Y around the star B.
Everything else equal, except for the rotation speed of both planets. Planet X rotates 20% faster than planet Y. The question is, which planet has a higher average surface temperature.
Edit: See discussion: