I want to kill my professor for giving me a bad grade, so here's my maxim: If you were born on November 1, 1985 ... you can kill your professor.

The answer I heard somewhere was that this line of reasoning was an application of the meta-maxim 'I will invent highly specific maxims to allow me to do whatever I want', which itself cannot be willed as a universal law without contradiction.

Edit: alternatively, the CI was intended to be a necessary condition, not a sufficient one. (disclaimer: I haven't read much Kant in the origional)