Just to add to the possibilities, I get numb toes on one foot but not the other. After some time experiencing this (and not figuring it out), I noticed that my toe nails were taking on a more flattish shape. I think that it's a matter of form and loose-fitting shoes - I think that I'm pointing my foot down too much, my foot is sliding forward and my toes are cramming in to the front of the shoe. I'm making an effort to consciously practice better form, but it's still pretty easy to fall in to bad form when tired.