Piston monostable circuit vertical with redstone block glitch

I have a link to a video download here :

https://dl.dropbox.com/s/mg4oqt52puagepr/Piston%20vertical%20glitch.avi?dl=1

Basically, I have a 1-Tick monostable circuit that causes a piston to drop the block. When facing vertical with a redstone block above it, the piston will freeze in the on position. When using an iron block or any other block, this does not occur. When placed horizontal, the piston behaves normally.