When you place a sticky piston and a redstone block above it and power the piston, once the signal from the sticky piston is out then the sticky piston will not retract (screen shots below)

What I expected to happen was...:
The piston to retract after the redstone signal was out

What actually happened was...:
The piston was still being powere

Steps to Reproduce:
1. Place a sticky piston facing up (i dont know if this also afects other directions)
2. Place a redstone block directly above the sticky piston RETRACTED
3. power the piston
4. The sticky piston will be in the active state, so the redstone block will be pushed up.
5. Remove the redstone signal from the sticky piston
6. Poow! The sticky piston will not retract.