So, I was building a 3x3 piston door, and I noticed that the top piston wasn't retracting. I tried updating the piston, and the blocks near it, but to no avail. Then I realized something must be powering it, so I broke the redstone blocks above it, and it worked. I also did some testing in single player, and the same thing kept happening.
Photo 1: the bug in all its glory (Singleplayer) http://tinyurl.com/onrwgmp
Photo 2: the place where I discovered it. Notice how the redstone block is missing on the right, and the piston is retracted. (Multiplayer): http://tinyurl.com/pq7cqqf