When you try to push a block with a normal piston through the worldborder, happens nothing. The block i outside of the worldborder but the Texture is still in front of the Piston.
But if you're trying to push a block with sticky piston through the the worldborder , it works. First is everything similar the the normal piston. But if you retrackt the piston, the piston gets glitchy and the block glitches through the worldborder.
You also can push dropped Items through the worldborder.
You can use this bug for very bad things. For example: On servers, griefers can now "destroy" expensive/rare blocks from players. Or they can push expensive/rare Items like a Dragon Egg through the worldborder.
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Thank you for your report!
We're actually already tracking this issue in MC-82010, so I've resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
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You can push a block through the worldborder with a sticky piston. But the block moves through the worldborder. After that there is no ghost block. And also Items can be pushed through the worldborder.
I cannot find an issue about items going through the worldboard. Feel free to edit this issue or make a new issue after you search.