if you'll put an item on a piston that is activating and reactivatings over and over again with redstone, then step back about 100-200 blocks away, log out of the world, come back in, go back to the piston machine, the blocks that is attatched to it might be duplicated. i only tested it with slime blocks.
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Mind attaching screenshots of the setup?
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the pictures represent the bug setup (the piston machine, backing away and logging off the world, going back in and go to the piston machine)
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Yes, I've noticed it too. Didn't happen before teh pre-releases. Now it's messing up my sugarcanefarm and mobfarm. Not only slime blocks are affected. You can mine away the blocks (won't get any drops) but they will re-appear when going x-blocks away. (survival)
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Same here. Duplicated blocks disappear after quitting and reloading.
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I observed this bug in 16w06a too. It can happen right under my eyes, no need to go x blocks away. When a sticky piston retracts, it pulls the real block back, but leaves a client side ghost block behind. When the piston extends again, this ghost can get get pushed further and pushes whatever is on its way too. But all this happens in client side only, so the mechanism doesn't really get messed up.
When this happens to a slime block contraption moved by a single piston, then every moving block that should leave empty block behind gets duplicated. It seems to never happen to just a subset of those blocks.