In 1.16.3 if you power a downward-facing piston from 1 block above, and 1 block to the side (ie. diagonally) with a Redstone block, it fires the piston.

I'm not sure if this is intended behaviour, but if you then remove the power source, it does not un-power the piston. This is definitely not intended.

Found by players on the LTT Network server - namely ExecutiveCactus