If there is a mod on you can claim this report as a folly on my part... LOL the piston and the slime block was adjacent to the ground.
crashes somewhere between(x 29999850 z29999850) and (x 29999840 z 29999840)- that is`nt said anywhere else,
as far as I can tell.
-Usefull information.
Did you *(1)* Activate command block with "/summon Arrow [(X) (Y) (Z)" (The place where the slime block will be, or over it)] *(2)* Activate a piston under the slime block. Then watch the arrow disappear as the slime block is pushed up.