In some situations, when dust powers off, pistons do not retract, even though they should have received a block update at the same time. (Hence this is not a duplicate of MC-108.) To reproduce:

1. stand on x-178.5, y12, z-1211.5 (other positions work, but I haven't found the pattern yet)
2. put a piston two blocks under you (not facing up) and dirt directly (one block) under you; when standing on the dirt, you should be at the position given in (1)
3. make a trail of dirt heading any which way and cover it in redstone dust
4. put a power source an even number of meters away – e.g. two dust and then a lever
5. toggle the power source a few times

expected behavior: piston extends and retracts
observed behavior: piston extends, but does not retract

In case my text description of the setup above isn't clear, I've attached some screenshots showing the setup and what happens after toggling the lever twice.