Bug: Hitting your head avoids fall damage.
Severity: Exploitable.
Congratulations to Simon for finding this one.
A lemming can survive a drop of any height if he hits his head on the ceiling at the start of the fall. For this to work, the ceiling must be EXACTLY 6 bricks off of the height of the floor you are jumping from. Any lower, and the lemming will not jump, any higher and he will not hit his head.
Level authors can mostly mitigate this by offsetting the floor or ceiling by half-a-brick height, it is not possible to dig or build half a brick of height, but it may be possible to align yourself by using a bomb crater, so the bug cannot be totally avoided.
See attached animated gif for a demonstration.