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.
Another great one is 'I always thought my cat had a staring problem. She does.' This plays on the uneasiness of being stared at by your pet in a way that makes it seem more than just normal feline behavior. It gives the sense that there might be something more sinister about the cat's stare.
One great 'imdb two sentence horror stories' could be 'I woke up in the middle of the night. There was a figure standing at the foot of my bed with no face.' Another might be 'She heard a scratching at the window. When she looked out, a hand with long claws reached in.'
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.