I did some testing and it appears that in 1.4.5. the block that a piston pushes upwards also powers the piston, so this locks it in place permanently.

What I expected to happen was...:
The piston would retract.

What actually happened was...:
It did not.

Steps to Reproduce:
1. Place a piston facing upwards (either kind)
2. place a block on the piston
3. power the piston a constant source (torch, lever)
4. place a lever on the block extended by the piston and turn it on.
5. Turn off the source directly powering the piston.