I came up with this one, remembering a simpsons (treehouse of horrors) episode where a witch makes the costumes people are wearing real (bart becomes a wolfman, ned flanderrs becomes a flower):
I decided to write my own story:
A witch showed up at a halloween party. Nobody really knew who she was, a lot of the guests were thinking maybe she crashed it? Finally the hosts asked her to leave.
"But why do I have to leave?" she asked.
"Because, like I don't even know who you are, and this is a private party".
"Well, that seems awfully selfish. I'll leave, but before I do, I'd like to give you and your guests a little gift. I'd also like to let you in on a little secret.. this isn't a costume, I'm really a witch. And I cast a spell apon you all! You will ALLLLL become that which your dressed tonight! heee heee heeeee!"
"Aren't you the cute couple, dressed as smurfs? Well I hope you like being one, because your going to be one for real!" And with that, they shrank down to only 6" tall, as the scurried inside a mushroom that grew from the carpet, which resembled a house.
"You, your not so bad, this is one of the rare times when my curse might actually be a gift!", and with that the man dressed as an undercover superman actually turned into superman, and with that he flew up through the ceiling into the night sky.
"And your a zombie I assume?"
"Yea, but don't change me into one, I don't want to eat brains!" she pleaded. But it was too late, the curse was already cast. The girl got very glassy eyed, and began walking slowly around the room with her hands up in the air, trying to find a victim she could devour the brains out of
"Super mario? Seriously? How is that even scary?"
"I wasn't going for scary, I was going for fun"
"Well you'll have plenty of time for fun, when you get transported into the TV and be part of a video game, hee hee heee!"
"And just what the prey-tell are you supposed to be?" she asked
"Duh, I"m a vagina!"
"Well, hope you like being one, because that's what your gonna be!", and with that his head turned into an actual fully working vagina. He stuck his fist inside, and he could feel the sensation. When he talked, his pussy lips moved like a mouth.
"Mmmph what the hell did you do to me?", he asked in a female voice
"And what are you supposed to be dressed up as miss?"
"I'm not a miss, I'm a dude"
"A man? dressed up as a girl? oh man, this is good"
"What.. wait, no.. you wouldn't!"
"Oh I think I would".. And with that, the man dressed as a girl found his body changing to match his costume. He lifted up his dress to feel his crotch, and confirm that he did indeed have a vagina as he looked up at the witch in horror.