Basically, when you extend a piston, it goes directly to the extended appearence, without the animation. Not true for retracting.
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Was able to recreate back in 13w06a, but now in 13w09b it looks fixed for me except for [this|MC-8264] case.
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Still able to reproduce it. If I power the piston by placing a redstone torch - no animation; via redstone wire - proper animation.
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MC-8184 might be the cause since no animation plays when using redstone blocks/torches.
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As predicted fixing MC-8184 fixed this too.
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It's a graphical glitch,and does it even annoy you?
Confirmed.