(Sticky) pistons are able to fire more than once per tick, which can lead to very weird results like redstone dust that is powered on and off more than once per tick (compare
Also, it allows you to move blocks more than one square in a single tick which looks like teleporting.
And it leads to weird visual artifacts.
I have attached a test world where you can reproduce the problem.
Place one block in the middle between the pistons and it will move through all 4 positions in a single tick.
Place two blocks diagonally to each other and they will swap places every tick.
Suggested fix: Pistons should behave like one-tick-repeaters and only check once per tick if they should be extended or not.
Note that this bug is limited to sticky pistons. Replace the pistons with normal ones, and it will behave better, although you will be probably affected by invisible blocks (