I found a few threads on the forum that talked about pistons getting stuck open. I encountered a similar problem today, and I think that I may have figured out what was causing it (Rather than necro-post on one of these threads, I decided to create a new topic with my findings).
I realized that I had a protection stone that covered the square that the piston extended into, BUT NOT the square that the piston's base sat on. This caused it to become 'stuck' after it extended. Removing the PS solved the problem!
Is this, perhaps, a problem that could be fixed so that players don't just have to work around the glitch?