This seems to be a somewhat inconsistent bug that isn't hard to replicate but may require some RNG factor as well. Whenever you crouch on a moving slime block and move near the edge, you may suddenly die inexplicably. The world reports "Player fell from a high place" as the cause of death. I believe that somehow the game is thinks the player has fallen from some incredible height that kills instantly regardless of armor, although I only had a full set of iron armor on at the time.

State at the time I discovered the glitch:
-Full and low health (I tested it multiple times)
-I was using a full set iron armor (on every test, as I didn't want it to despawn)
-I was crouched on a 2x2 slimeblock platform on a flying machine
-moving to the center of the platform killed consistently, moving to the sides or edges don't seem to insta-kill as often.
-Crouching on the edges and moving slightly occasionally does 3-4 damage (with full iron gear)