Unless the blade was already fatigued or highly brittle it's not probable at all. Weak points in any sword are at the hilt and tang due to changes in the cut, hardening and point of transition. But as I said, it could happen if the blade was already basically broken- I have had tools break due to over hardening, I still would highly doubt this though as most of the time the force would need to be twisting and against the grain of the steel.