On any fast extending piston elevator setup on halfslabs you never touch the ground while going upwards resulting in accumulating falldamage, as seen here http://youtu.be/snyRGMOnZh8?t=2m24s

One Possible solution to this and many other bugs related to fall damage would be to make the fall damage only depending on the vertical velocity an entity has when hitting a solid surface,or to reset the fall height when being moved upwards by a piston.