Downward-facing pistons powered by a solid block above with redstone dust on top sometimes don't retract if the dust had an even signal strength. For some reason whenever pistons are powered with an odd signal strength they function as expected.

See screenshot for setup, should be easily reproducible. Pistons are one block apart to stop them updating each other. Pressing a button above a piston works as expected, but pressing a button above a gap results in all pistons extending but only some retracting. The same pistons retract each time, so this is deterministic, but there doesn't seem to be a clear pattern when building this structure in a different location or direction.