Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
I think 'The clock stopped at midnight. Then I heard footsteps.' is quite scary. It gives this feeling of something ominous starting right at a spooky time.
He looked in the mirror. His reflection had a sinister smile that he didn't make. Well, it's really scary because the mirror is supposed to reflect our true selves. But when the reflection has a mind of its own, like showing a sinister smile that doesn't match what we are doing, it creates a sense of unease and horror. It makes you question what is real and what is not. Is there something wrong with the mirror? Or is there something wrong with you? It plays with your mind and makes you feel like you are not in control. And that's the essence of horror in this simple two - sentence story.
Some two - sentence horror stories are truly spine - chilling. For example, 'I woke up to a noise in the kitchen. When I got there, all the knives were floating.' The simplicity of the setting and the unexpected, supernatural element make it effective. Another one could be 'She heard a baby crying in the attic. She doesn't have a baby.' It plays on our primal fears of the unknown and the unexpected in a very short space, creating a sense of unease and horror.