Using pistons, I have noticed that when powering them, then placing a sign adjacent to the powered piston head, they will stay activated even when there is no more power source. Steps to re-create:
1.Place down a piston with a lever nest to it(left or right of the piston head so it's powering the piston).
2.Activate the lever.
3.Place a sign adjacent to the extended head of the piston.
4.Either destroy or turn off the lever.

NOTE: THIS ONLY WORKS ON A SERVER RUNNING 1.4.6, SO, I CAN NOT PROVIDE A SCREENSHOT,OR CONFIRMATION OF WORKING