block update detector (like in picture 1 sometimes disapear)

What I expected to happen was...:
When I build the block update detector (in picture) piston will stay in that and block don't disappear

What actually happened was...:
piston is disappear and come back.disappear and come back...

Steps to Reproduce:
1. build that block update detector
2. start it and watch ,you will see sometime piston disappear

or watch this
http://www.youtube.com/watch?v=1du3yywmflg