Blocks attached to slime blocks can create ghost blocks

16w39c reproduction case in MC-108310.

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Blocks that are attached to slime blocks can create ghosts blocks if a piston that's receiving a 1-tick pulse pushes the slime block
This very short video explains everything:
https://www.youtube.com/watch?v=HH_FlU6wZW0

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To reproduce, download the attached test world (bug_world_MC-54026.zip), press the button to let the machine go up, and then press the upper button for it to go down. Repeat until ghost blocks occur.

World with example contraption provided. It may be necessary to increase the height of the thing to get it to bug out. I've only ever seen it occur on the way down.

There is a button on the bottom for it to go up as well as at the top to make it go down. Movement is triggered by block updates so if you do something to block its path and it stops moving, just place and break a block near the BUD switch and it will start moving again.

Matt D. on SPIGOT-3015

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Suggested fix by [Mojang] Gnembon can be found in fix_pistonGhostBlocks.PNG; explanation can be found in this comment