I lost my hardcore world to this bug. I was building a mob farm, and had scaffolding laid out in the drop chamber to protect against accidental fall.  I tested it up to 20 blocks, and took no damage falling onto it, as expected.  When I finished the farm, I went to drop down the tube, which was now 28 blocks, and died upon landing.  

I created a test world to see what had happened.  To duplicate the bug, I created a smalll landing platform on the ground of hoppers with scaffolding on top of them, and another platform of just scaffolding.  Between them, I built a tower of scaffolding at increasing heights. Falling onto either platform resulted in no damage whatsoever up to a differential height of 27 blocks.  At 28 blocks, falling onto the hopper/scaffolding platform results in death. 

I am, needless to say, furious at losing a hardcore world due to a bug.