Occasionally (unable to reproduce reliably) pistons get stuck in the closed position no matter how many times you power and unpower it, and give it block updates.
I have determined the cause: The Piston Extension block (I believe it is ID #36) is the block which is used during the extending/retracting of a piston. After the piston is done moving, the Piston Extension should be overwritten by air or the block being pushed, however sometimes it stays. Because it is unpushable, the piston gets stuck. The piston moves once you break the Piston Extension.
I believe this bug is caused when the game crashes while the piston is moving. This is just my theory. Please fix this, it is causing redstone machines to stop working.

As you can see in the last image, once you break the Piston Extension in front of the piston it opens the piston