While building on scaffolding in my survival world I took damage unexpectedly.  The scaffold base was at approx Y=-20 and I was standing at Y=2.

In a test world I created a similar setup by crudely replacing the top part of a chunk with air blocks (in my survival world I was digging out a whole chunk so it's a similar situation). I've attached the test world I can reproduce this on (at least on Windows).  I have not yet tried other platforms but can do so at some point if needed (Xbox and iOS at least).  The test area is in the 0/0 chunk in the attached world.

My observations:

• When standing on scaffolding at Y=2 or Y=1, you will seemingly randomly take damage.  Sometimes trying to place a block (e.g. extending scaffolding) will trigger it, other times it appears more random.
• This appears to relate to the absolute position of the top, rather than the relative height between floor and top.  In the test world there is a scaffold up to Y=3 starting one block higher up, so the same relative height, I cannot reproduce the problem there.  I have not however tried every possible height.
• The damage taken appears to be consistent with how much fall damage you would take if you fell off.  There are various scaffolds set up in the test world with a different height, but with the top at Y=2, one has a 4 block drop and the unexpected damage is half a heart as expected.  The death message if you die is 'fell from a high place'.
• Perhaps even more strangely, if you eat while standing on the scaffold at Y=1 or Y=2, you will fall through the scaffold after eating, taking the expected fall damage.
• If you eat while standing inside the scaffold tower, at Y=2 or Y=1, you descend one block down the tower.  Once you get to Y=0 this no longer happens.  This doesn't happen at any other height in the tower (-20 to 0, haven't tested other levels).
• Crouching does not appear to prevent either the unexpected damage, or falling through when eating.

This all seems very odd.  Can anyone else reproduce this?  If it's just me I'm not sure what I've done to cause this.