When a piston is turned on at the same time another piston is turned on diagonally forward and left or right (I haven't tested up or down), the piston closest to the signal source will appear to be both extended and retracted at the same time, and will not be fixed until the player breaks the extended part of the piston.
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Duplicate of MC-127 , please use the search function to see if your bug has already been submitted. Currently over 42% of tickets are being closed as duplicate.