This isn't only graphical glitch. The piston arm is actually extended and it appears 'painted in the front part of the piston body as well, but if you try walking through the piston arm, it won't let you, therefor, isn't graphical glitch only. I messed with it, and it's a directional glitch, which means that only happens when the piston is facing some directions. In this case, it may/will happen when the piston is facing: F=0(South); F=2(North).
This causes an ineffective auto-farm and a lot other issues.
Can happen with both Normal and Sticky Pistons.
And it's more frequent, if not every time, when the piston you pretend to extend is pushing something (like Sugar Canes).