I had some dupes digging out copper under water. One of them was on a square which should have caused him to fall 2x squares once he dug it, but instead he fell 5x squares and clipped through 2x squares of rock and was "stuck." My dupes heroically tried to dig him out, but he drowned (suffocated) before they could get at his body.
Have a dupe dig on a rock that is a few squares above the ground causing him to fall once he has finished. See if he clips through the rock.