In this image, the pistons should accept any power from the green blocks, check to see if the orange blocks would notify it before accepting power from them, and not accept power from any of the stone blocks:

What actually happened was...:
Pistons extend when blocks are providing power to the block above them, even if they don't update the piston. In this case, the piston must be updated by another method in order to extend or retract.
Example of odd effects this has: http://www.youtube.com/watch?v=JqU0Kmb_bFA
Steps to Reproduce:
1. Place a piston.
2. Put a block on top of it.
3. Put another block on any exposed face of that block.
4. Put a lever on the block you just placed and flip it.
5. Notice that the piston doesn't extend.
6. Place or break any block directly next to the piston.
7. Notice that the piston extends.
8. Flip the lever off.
9. Notice that the piston did not retract.
10. Place or break another block directly next to the piston.
11. Notice that the piston retracts.