Final answer:
In the 'My Little Pony: Friendship is Magic' show, Pinkie Pie's cutie mark was switched with Rarity's during the episode 'Magical Mystery Cure'.
Step-by-step explanation:
The character Pinkie Pie from the animated TV show My Little Pony: Friendship is Magic had her cutie mark inadvertently switched with another character's due to a spell gone wrong in the storyline. When Pinkie Pie's cutie mark was switched, she ended up with Rarity's cutie mark. This event occurs in the episode titled "Magical Mystery Cure," which is part of the show's third season. The change in cutie marks causes each of the mane 6 characters to attempt to live the life that corresponds with their new mark, with comical and heartfelt results, as they are not suited to their new roles.