sealcat prompted me with Harry Potter and Severus Snape (wildcard genre): cockatrice feathers, Serpent's Anklet, and the Idiotic Unicorn.
Severus' Own Idiot (PG-13; Snarry; 375 words): Severus doesn't appreciate Harry's attempt to mix work and play.
Severus affixed the Serpent's Anklet to Harry and watched in annoyance as the transformation stopped.
"Oh, that's much better."
"Is it?" asked Severus, glaring at Harry's breasts.
"Don't worry. I'm sure the effect won't last long. Why don't we go upstairs?"
"Harry, the Idiotic Unicorn's run by a virgin sect. There is no 'upstairs'."
"Just because the Order of the Sexless doesn't have any is no reason we can't. Besides, they're harmless."
"If that's true, then where did your cock go?"
"So they experiment. It's my own fault for drinking the lager. Cockatrice-hunting's thirsty work!"
"You idiotic Auror! Weasley didn't drink the lager!"
Harry grinned. "Yeah, well, you did say things had got a little stale, lately."
"Where is that damned dragon-cock you brought down?" Severus demanded.
"In the back. Why?"
"Accio cockatrice feathers!" Severus cast, seizing them when they levitated close enough and setting them alight. "Inhale," he ordered Harry, thrusting them forward.
Harry did so, sneezed, and gasped as his masculine features returned. "Well, fine, so I was wrong about your tastes."
Severus shook his head. "You weren't, but it was creepy."
Harry raised an eyebrow and reached down to remove the Serpent's Anklet. "If it was so creepy, how is it you just happened to have this on hand?"
"Obviously," continued Harry, "you've worked with sex-alteration spells before, or you wouldn't have something that maintains the—"
"If you want to try that sort of change for a lark," Severus replied, "we'll do it properly—with Polyjuice."
Harry crossed his arms. "And just where would I get the hair?"
"From anyone who doesn't look like your mother!"
Harry's eyes widened. "Oh, fuck. I didn't think."
"No, you didn't. And if you think I'm taking you to bed after this little stunt of yours, you are greatly mistaken. For your information, I only put the Serpent's Anklet on you because Unicorn Lager causes rapid transformation between the male and female form, which, if allowed to go on too long, can result in a permanent change of sex!"
"Master Horgath didn't tell me that!"
"Master Horgath called you here to defeat a rampaging cockatrice," Severus said. "What does that tell you about his 'harmlessness'?"
Shrugging, Harry asked, "Whose hair, then?"
Severus shook his head.