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.