Put the summary of the bug you're having here

I was making a double piston extender for a vertical piston door I was making for my parkour map, but then I notiched it was dispearing.
Describe what you thought should happen here

What actually happened was...:
It was a bug and it causes the piston to disapear

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. I have a video unlisted on youtube, here is the link http://www.youtube.com/watch?v=Zc8SPILHVXk&feature=youtu.be
3. I used this tutorial to help me make the door http://www.youtube.com/watch?v=IP3Em1lcbrI