A Mistake is an item that is created when Mario mixes ingredients that do not result in an applicable recipe. It restores 1 HP and 1 FP. In Paper Mario: The Thousand-Year Door and Super Paper Mario, making a Mistake counts towards finding all the recipes.
In Super Paper Mario, there are two different Mistake items. The usual Mistake restores 1 HP and cures poison, while the other Mistake has a purple fish skeleton, and can only be made using a Sleepy Sheep. The purple Mistake will put enemies to sleep when used.