Redstone machines have sometimes used a design similar looking to this tiny contraption. Redstone torches now powering the block below it and activating pistons is not good and can break such things.
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This is normal behavior. The torch is powering (weakly, but nonetheless) the block next to it and above the piston, causing the piston to respond to the torch's power state.
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Duplicate of: MC-108 - Please use the search function to see if your bug has already been submitted.
Currently over 45% of tickets are being closed as duplicate.
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Hawk is correct. Pistons will activate if any adjacent block is powered, however weakly.