Looks like if the Piston is trying to extend and retract at the same time, while receiving indirect signal (being BUD) from a moving Redstone Block and receieving block update, it creates two arms, both with proper hitboxes, one is extended, one is retracted. This piston won't work until you break the extended arm that preventing the retracted one from extending.
It can be a duplication of MC-127, but this bug is NOT a directional one and has different causes. Also, the second arm is NOT a ghost-block, it has a hitbox and does not dissapear after reloading the world.
The little useless (is it? :]) contraption in the first screenshot extends all upper pistons consecutively by turning the lever on and retracts all of them at once by turning it off. The lower line of pistons works as BUDs and updates itself by moving wool blocks. Lower pistons receive indirect power from redstone blocks above.
After the first retraction second piston in the lower line retracts and extends very fast and produce this bug.
Also, this line of pistons can be continued, causing bug on all of the lower ones, except the first one, but not always.
Another way of reproduction found by Rlizeify (see the second picture). Somehow the repeaters are crucial
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Duplicate of MC-127.
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Tails, wait. I don't think so. I was updating this page, describing why it is not a duplicate and directly related to the new redstone blocks, when you marked it as dupe :c
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All piston bugs related to a piston beeing extended and retracted at the same time are summarized in MC-127. If you have found another way of reproducing it, please leave a comment there.
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Alllrighty then