When a simple line of Redstone gets unpowered a piston may stay extended. It seems the redstone on the iron block gets unpowered before the redstone on the blue wool. This causes the piston to get diagonally powered thus not retracting. How ever the redstone on the blue block should also be/get unpowered as well. Which does happen but apparently not in time for the piston to notice it.

There shouldn't be any delay in the blue block getting unpowered when the iron block gets unpowered.

Something which is very interesting is that it only happens when I press one of the buttons on the red wool. (The red wool itself does not cause this. I have marked the buttons which cased it with red wool.) If the line got powered form an other location it seems that the piston doesn’t get diagonally powered. And therefore retracts as expected.

As I said the piston gets powered diagonally. So if you would change the blue wool with a slab the problem doesn’t exist. However this isn't always a option in a Redstone build.

A final interesting thing is that it is location based. As you can see in the images I build two tests and the wool markings vary. However if you create a new world and build it in the same spot it will stay the same. Piston location x=944 y=65 z=-564 the piston is facing west.

The isn't just a thing that happens to pistons. It also works on all block affected by MC-108. (Please mod's do NOT mark this a another MC-108 dup. MC-108 is involved but this isn't a dup. As this should always update the piston. And cause it to retract.)