What I expected
Piston would do nothing.
What actually happened
The Piston extended

To re-create the bug:
1. Take any block* and place it anywhere.
2. Place a redstone torch on top of said block.
3. Place either a normal Piston, a sticky Piston, or a Dispenser next to the placed block, not next to the redstone torch.

And vióla, that's how you replicate it.
EDIT * Block can be either transparent or solid.