Invalid Block Updates

The observer updates when it is invalid.

TO REPRODUCE:

1. Place an observer facing downwards in the air.
2. Put a sticky piston on the 'outputting' part of the observer.
3. Put a string of 3 pieces of redstone (one being underneath the observer face).
4. Break the second piece of redstone and see that the piston extends and retracts.

It is an invalid update.