The Simpsons: 15 Huge Guest Stars & Cameos You Forgot

15. Frankie Muniz

As far as child TV stars go, there weren't many bigger than Frankie Muniz at the height of his popularity. The star of Malcolm in the Middle took the world by storm at just 14 years old, and made guest appearances on the likes of Lizzie McGuire and Sabrina the Teenage Witch.

He also crossed over into the world of animated TV in 2001, just one year after the debut of the show that made him famous, making his one and only Springfield appearance in 'Trilogy of Error'.

The episode was broken into Homer's Day, Bart's Day, and Lisa's Day, with Muniz playing a part in the latter. With Lisa having to hitch a ride to school with Krusty and Mr. Teeny, she was accidentally taken to West Springfield Elementary School, where she met and immediately fell in love with Thelonious.

Like Malcolm, Thelonious was a nerd and something of an outcast, and found the closest thing he had to a friend in Lisa. The two just spun in each others' arms for hours before Lisa had to leave to find her own school, and though she hated leaving Muniz's character, neither of them could ever sacrifice good grades for romance.