As you read this thread, you find, as others have noted, that the plausible posts are much scarier than the fantasies.
But the post that will truly haunt you - the one that will forever creep around the edges of your consciousness and grasp hold of your imagination at the strangest moments, never fully releasing it no matter how you laugh, or shudder, or shake your head - isn’t one that could happen… it’s the one that will.
Make that last bit “…from under the covers.” and I think you have yourself a winner. 
He woke up in a strange bed, with the odd feeling that something was missing.
As he looked downwards he screamed when he saw that it was.
One we got in my high school English class:
He was mowing the lawn. He did not see the nest of baby bunnies.
He looked at the remains of the burned wasp nest and noted the tiny charred bodies arranged into letters: “vingince” scrawled across the gravel drive. As the black cloud descended on him, he realized: Wasps can’t spell.
That’s good!
Well, I was divinely inspired by a recent shrub-trimming experience. :eek:
Your shrubs can spell?
What’s the story here?
Resubmitted:
“It’s OK, kids” he said as the two children climbed into their parent’s bed after waking up in the middle of the night, “There’s nothing to be scared of.”
“I disagree.” came a strange voice from under the covers.
I prefer to ratchet it up a bit:
Being a heartless bastard, he didn’t even hesitate when he saw the nest of baby bunnies while pushing the lawnmower through the long grass. Later that night, though, as he drunkenly ambled home from the pub, he found his footsteps dogged by an odd multiple echo, and when he looked around in the gloom, he saw hundreds of flares of pale white in the darkness, for all the world like the flash of a cottontail…
If they could spell they probably would’ve been writing out “Please don’t kill us!” with all their sheared ends. 
The nest of stinging mud daubers* that I disturbed, however…I don’t know if they can spell, but the can definitely say something close to “Kill the intruder!” in a drone-y, dive-bomb-y sort of way.
*You’ve always heard mud daubers don’t sting, you say? Yes. So have I, and we were wrong. In all fairness, however, they are pretty wimpy stingers, and they aren’t all that dedicated to it.
The newest and most pernicious plague of them all was transmitted not by contact, not by infected air or water, not by fleas or ticks, but an entirely new and unanticipated means: through the very ether of the Internet.
And by the time you so much as read the words of someone infected with it, it was alreaaosidhfpa oi a[oidfn apoia [oaijs[dfco (* 09we8r cxlzvck
Pepper asked the question we all have at one time or another, “Daddy, why do we have to live underground?”
He put his beer back down on the table and replied, “The shrubs learned to spell”
Then a quick, involuntary laugh escaped his throat. “Well, not very well anyway.”
Love it.
“Time heals all wounds, my ass!” he thought bitterly as another one of his fingers fell off. Then again, the talon beginning to poke through at the stub where his thumb used to be held some promise…
[Yes, cheating on the number of sentences sue me]
“Hey Mommy look at my scary Halloween costume!”
“Wow! That is indeed very scary sweetie!”
“Mommy I’m standing over here…”
This is a sequential thread which also makes a good two line horror story:
For future historians: pithy statements that might make more sense later. Much later.
Escaped python kills 2 kids - something doesn’t feel right here.
You think of the greatest two sentence horror story of all time. Moments before you try posting it, the thread is locked.
Ha. When I read yours, I thought “Hey, that sounds like it could be a Chopping Block strip.” So apparently you made it work quite well.
Also, I think the “Frank?” reference is to this strip, especially given the author’s comment: