In multiplayer,if you place a piston with a redstone block on him, a block on the side of this one with a redstone powder, then you destroy the last one, the piston stay extended until you break the redstone block.
But, if you destroy the piston, and you place an other one on the same block, the new one will be powered.
I am sorry about my bad english 
- duplicates
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MC-108 Droppers, Dispensers and Pistons activate when blocks are providing power diagonally or two blocks above
- Resolved