Sudden Switch! The Curse of the Contrary Jewel (Shampoo no hyouhen! Hanten houju no wazawai) 30 years old

The story, originally published in Ranma ½ volume 22, features a brooch that makes Shampoo feel the opposite of her true feelings for Ranma. Ranma begins to panic at the thought of losing one of his fiancees, and plans to tell Shampoo that he loves her. Forseeing this, Cologne invites all his friends, so once he confesses, he'll be unable to back out of their engagement. This benefits Ryoga and Kuno as well, as they plan to sweep Akane away once Ranma makes his supposed feelings for Shampoo public.