A badly abused cross peen hammer probably use just as was previously stated as a wedge, quite possibly as a splitting wedge until the handle finally gave up the ghost then used without a handle even after the slug in the eye crumbled out, Once the eye began deforming it would be very hard to get a straight hit on it. So very likely the hammer head was non tempered and only moderately hardened the eye may have failed in one final hard striking blow