Minecraft 18w07a, Minecraft 18w08b, Minecraft 18w09a, Minecraft 18w10d, Minecraft 18w11a
Set up two pistons with four blocks in between.
Put water in the center two blocks (one block above the pistons)
Put Magma blocks under the water.
Put soul sand in front of the pistons.
Push both pistons at the same time.
One of the pistons gets pushed back.
There is an unknown block appearing for a few frames. (see attachment)
We tested this on East-west orientation.