In previous versions of Minecraft, this bug did not occur. Here is how it worked: Take a piston and put 13 blocks in front of it's face. Then place a power source next to the piston. Then remove one of the 13 blocks excluding the one touching the piston. After this, the piston should extend due to it being powered by the power source, and there are only 12 blocks in front of it, but it doesn't. The piston will extend if it is given a block update however.