This rarely happens, a piston or sticky piston stays powered though there is no physical interactions with redstone. 

How to reproduce:

1. Place a redstone block.
2. Place a sticky or normal piston underneath it (facing any direction) (on top or on side won't work).
3. Destroy the redstone block or move it away.
4. The piston might be activated and will push even though in F3 mode, it says unpowered.