When powering a piston (both normal and sticky, facing any direction) via redstone wire on a solid block above, if the power level of the redstone is EVEN (2, 4, 6, 8, 10, 12, 14) AND both the X and Y coordinates of the piston are even as well (but not the Z coordinate as far as I can tell), then when the redstone wire activates and deactivates, the piston will remain extended until updated as though powered through quasi-connectivity.  As far as I can tell, this bug only seems to work if the piston is also being powered via QC.

How to reproduce:

1. Place a sticky piston somewhere such that it's y coordinate and x coordinate are a multiple of 2 (positive or negative)
2. Place a block on top of it, and redstone dust on top of that, then run that redstone line out in any direction, and power it such that the redstone dust immediately above the piston has a power level that is a multiple of 2.
3. Unpower the redstone. Piston does not retract until updated.