When a piston extends for a short amount of time it sometimes shows a texture on one face of the piston that it should not be showing. See screenshot. No functionality is lost.
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This Effect is easy to see when hooking a redstone pulser up to a piston.

Thought as much, but when I found it I had simply been adding a lot of random bits and pieces then noticed it

Duplicate of 136? Does it happen while extending, or while retracting?
On retracting.... it switches the top graphic of the piston base block to the top of the piston top either type of piston.
Yes duplicate of https://mojang.atlassian.net/browse/MC-136