Pistons (both standard and sticky) can remain extended when not powered.
If you power a piston from above, then destroy the redstone powering it, it will remain extended in some circumstances where it does not have anything powering it. (see attached video for clarity). I have also seen this happen when switching power off to the piston, if there is other powered redstone nearby.
VIDEO: http://youtu.be/mWb6oj2TxyY

The video was initially created for a Bukkit ticket, before finding out the issue existed outside of bukkit.