Everyday precisely at noon on the sidewalk behind the beach a grumpy, suspicious old woman walks by. What nobody knows is: She's the Witch of True Lies -- whatever you convince her about, is. What she believes becomes reality.
Basically, eventually... a reality editing story that properly abused could be a bit like jennie story technically perhaps, but with extra steps and limitations.
Being a witch she knows awful much about anyone and anything (don't ask me how), but by far not everything. And She may not at all be very interested in what you have to tell, but may react to things or events she sees. And she will tend to believe what she sees, but how she interprets it is what matters.
It is also possible, but not a given, that the witch has her own agenda and goals, but she can't lie to herself too well, so she cannot edit reality quite on her own.
And long before getting there, there's a discovery phase where it may not at all necessary be clear what's exactly is going on. Or it could be used as just that, a one time bizarre event that brings absurd change into the world.
The example is from a quick wank fantasy, so it's not very coherent a setting, or ever the best for the concept as stated, but that's where it comes from...
...as had been become customary as of lately, girl jumped out of her mother's car to spend an hour or two on the beach while mom will do her things in the town. Girl changed into her swimsuit by the car waved mom goodbye and ran to the beach. The beach was still empty, the only few other people half a mile away, and feeling naughty, girl decided to go for a quick skinny dip. She quickly pulled her swimsuit off, dropped on the bench by the sidewalk and ran over the sand into the sea.
It just so happened that her returning forom that there was this old woman on the sidewalk. Witch had noticed the girl already, so it was too late to hide. For whatever reason the old woman picked up the girl's swimsuit, asking if it is hers.
Girl thought she's in trouble, and decided it could lessen the stranger's leverage on her if she will deny it, so she claimed that no, it isn't her swimsuit, and answering follow up questions, that she indeed arrived to the beach nude, as she now always do, and her mother knows and is okay with that, if not even had encouraged her to do so, and may or not have made some more unnecessary claims.
The witch went away, but the swimsuit was gone too. It had never existed. Only our girl now has memories of two competing realities, one where she knows she had the swimsuit and must wear it, and another she made up convincing the witch it was okay for her to be nude, only she knows she lied, even if the old woman appeared to believe her. She yet doesn't know her lies had become true...
