A piston facing up will accept power from the block its pushing when the piston is extended and the block it is pushing receives power.
This occurs both on servers and single player.
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[media]

uh, this is intended

perhaps, but I have a redstone project that because bug make it impossible to do with making my redstone messy and un-compact

Im building it on cubehamsters server

71st duplicate of MC-108, please use the search function to see if your bug has already been submitted. Currently over 58% of tickets are being closed as duplicate.