So I killed my first PC with fall damage. Just doing the standard 1d6/10ft.
I had the party climbing down a rope into the ravine that holds the Sunless Citadel. I asked what order they climb down the rope in. In that moment I think they assumed that I was setting up for combat at the end of the rope, so the order ended up being Paladin, Barbarian, Sorcerer, Ranger, Wizard.
I was trying to set up a dramatic ending to the session, and the mercenary captain who led them to the ravine looked over the ledge, into the wizards eyes and said "If you don't come back with the macguffin, don't bother coming back at all" and then cut the rope. Like I said, I wanted it to be a dramatic, burning the boats type of ordeal. I said the paladin takes 1d6 damage, falling from the bottom of the rope, and each player up takes an additional die.
For most of the party the damage wasn't too bad. I think the first three players collectively took less damage than the wizard did, but she rolled really well.. Or really poorly, depending on how full you think the glass is. I think I also over estimated how many HPs a level 3 caster has. She rolled close to, if not more than 200% of her HP total. She asked if she was dead, and I said I didn't know, and that we'd talk about it.
I think I'm going to keep the 1d6 rule for the time being... But the lesson I learned was to not allow dice rolls to determine cinematic damage.