if you make a redstone wire on top of a line of blocks at least 12 blocks long and put a piston at one end and then power and unpower the other end, the piston extends when powered, but doesn't retract when unpowered.
to replicate:
build what is shown in 2015-04-07_22.46.10.png and turn the lever on and off
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Dupe of MC-11193

That's a BUD switch. Place a block next to the piston to retract it. It's happened for a long time now!!