Edit: Fixed as of 0.15.3 or 0.15.4
on occasion, when a block pushes a different block that is simultaneously pulled by a slime block, the block that pushed will cease to exist. Here is a diagram:
> = piston s = slime
0 = block x = vanishing block
Power the pistons simultaneously, and the x block might disappear
This only happens at certain points in the world, and those points change at reload. A simple way to recreate this bug is to make the flying machine shown in the picture, and wait for it to break. The flying machine will bounce back and forth between the two bedrock until one of the observers disappear. If this fails to happen, reload the world, and try again. It shouldn't take too long
This does not duplicate MCPE-14654 as any block can disapear, not just a piston.
This does not duplicate MCPE-15268 as the blocks are permanatly gone, not just temporaraly.