Break the piston arm

Break the piston arm

What I expected to happen was...:
That piston would break with the arm

What actually happened was...:
The arm of the piston broke, But the actual piston block remained.
Screenshot: http://tinypic.com/r/24ovx39/8

Steps to Reproduce:
1. Build a setup so the piston is constantly undergoing Block Updates.
2. This is the setup i used: http://tinypic.com/r/2aoiyp/8
3. Put a block on the end of the piston arm
4.The piston should constantly be extending and retracting.
5. Walk up to the piston sideways, and left click the arm of the piston.
6. Sometimes the block on the end will break instead of the arm. if this happens, just try again.

For some reason, This will only occur when the piston is receiving updates. A clock does not work.