for me: probably like what happened to Mrs. Weasley, it would just keep changing into all my friends and family laying there dead. murdered.
or a little child who has drowned and is bloated and blue.
or a really horrible car accident scene, car accidents are my worst nightmare. that and being kidnapped by creeps.
centipedes are scary too.
speaking of scary stuff. sometimes my friends decide to give me a heart attack by snatching my keys and sneaking into my car. waiting for me to get inside, when it's dark outside, and giving me the biggest fright of my life.