Here's mine, a cursed magic item for B/X D&D.
Cursed Pumpkin Pie Spice
(Miscellaneous Magic Item, Cursed)
This magical mixture of ginger, cinnamon, allspice, clove, and nutmeg smells irresistably sweet. It's typically found in a small metal container about the size and shape of a tinderbox, with a recipe for "Unbeatable Pumpkin Pie" written on the label (the DM is encouraged to use their favorite real-world pumpkin pie recipe for this label).
When found, the box contains enough spice for 1d6 uses. This delicious mixture of spices can be used in any recipe, and the food that this spice is added to will heal 1 hit point when eaten. But! if the recipe for "Unbeatable Pumpkin Pie" is followed and the spice is added, it will create a monster: an Animated Pumpkin Pie Filling! As soon as the spices are added to the custard mix, it begins to boil and seethe, growing rapidly until it forms a 10' x 10' puddle of sticky, boiling hot, and delicious custard. It attacks immediately and fights until destroyed.
The Animated Pumpkin Pie Filling uses the stats for a Lava Ooze (Rules Cyclopedia, p.188) except as noted here: Each hit does 4d6 damage and leaves a coating of delicious, sweet-smelling Pumpkin Pie Spice that lasts for 1d4 rounds thereafter. While coated with this spice mixture, the creature doesn't take ongoing heat damage but must save vs. spells every round or become confused, forgetting that they are under attack and becoming unwilling to do anything except sniff around looking for a delicious pumpkin pie that they are convinced is nearby. Multiple hits on a single opponent will extend the duration of this curse. A Remove Curse spell cast on the ooze will destroy it instantly.