Their brevity. 'One liner horror stories' are effective because they get straight to the point. For example, 'The mirror showed a face that wasn't mine.' There's no need for long build - ups. It immediately plants a disturbing image in your mind.
Here's one: 'The old doll's eyes follow me around the room.' Dolls are often associated with unease in horror. The idea that the doll's eyes can move and follow you creates a feeling of being watched. It taps into the fear of the inanimate coming to life and having an awareness, which is a common and disturbing horror trope.
There was a knock on the door at midnight. When I opened it, a cold wind blew and a voice said, 'Let me in.' I knew it was a ghost. This story gives me chills every time I think about it. It makes you wonder what would have happened if I had let the 'thing' in. And it plays on the fear of the unknown, the idea that there could be something lurking outside, waiting for an invitation to cross the threshold into our safe space. It's simple yet very effective in creating a sense of horror.
I told my wife she was drawing her eyebrows too high. She looked surprised. This is a simple but humorous one liner. The unexpected reaction of the wife after being told about her eyebrows makes it funny. It's a very common situation in daily life where small remarks can lead to funny moments.