That Sinking Feeling...

...When your protagonist is eaten by bears, and there's nothing you can do but narrate the incident; narrate the way the bear's slavering mouths clamp down again and again upon your defenseless protagonist's arms and legs, the sound of the bear's claws, rasping in the dirt while they tug at your protagonist's actual heartstrings, and finally, the smell and the texture of their mouldering scat as they walk away and leave your protagonist - now wholly digested - to decompose in little piles in the forest.

I haven't had this feeling before, but I'm sure someone has. Or maybe not. Either way, there's nothing more gut-wrenching than writing an inevitable scene.