When a piston is activated when a flowing water block is infront NOT a source block, the piston extends but the piston head is still where it would be if unactivated. So there is now two piston head one extended and one that isnt
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duplicates
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selbram this is different the piston head isnt flat ill add a picture
Re-opened pending pic from Chris B
This is probably related to the same visual glitch in mc-136.