While a piston is retracting, there is a visual glitch, see attached screenshot ("piston.png"). The wooden part which belongs on the end of the piston arm is painted on the piston base, too. On the screenshot, the green sticky blob should not be visible at all because it should be facing the stone block.
Bug exists with normal pistons as well as sticky ones.
See "expected.png" for how it should look like (screenshot made while piston was extending)