"Two pieces of matter cannot occupy the same space at the same time."

Well, looks like they can.
This happens the most with me in decapitation kicks, I have one that works just fine, but if I mess around with it, I will eventually come up with this, any ideas why?
It's annoying.

Such things happen is on of the colliding bodyparts' speed is too high. I tried to edit your replay a bit by slowing the leg and got the decap easily.
So yeah, this is rather annoying, but I suppose there's nothing we can do about it.