There are a few things in 5e that deal automatic damage with few options for defense. Magic Missile can be stopped by Counterspell or if the target has the Shield spell. Heat Metal is automatic damage (again, stoppable by Counterspell). That's what I've got off the top of my head - I'm probably missing a couple of other automatic damage, but they're few and far between. Most things that would light you on fire (which would then deal some automatic damage if you don't put it out) give you a chance to save against them.
Good point about Heat Metal dealing damage that has no saving throw. Still, it can be stopped by Counterspell, as you said, and is a Concentration spell, which can be broken, and Magic Missile doing unstoppable damage but can be stopped by Counterspell and Shield.
I'll bet that neither Counterspell nor Shield can stop the mighty medieval molotov cocktails' surface damage, though. Nothing in the game can save you from eating that guaranteed automatic damage.
