10 Best Celebrity Cameos In How I Met Your Mother

10. Laura Prepon

Laura Prepon bounced from one iconic sitcom to another.

After starring as Donna Pinciotti in all 200 episodes of That '70s show, Prepon popped up for a few episodes in How I Met Your Mother Season 4 as Karen, Ted's college girlfriend.

Lily and Marshall HATE Karen, with Marshall stating that if they got back together it would be the fourth worst thing in the world, and it's easy to see why. Prepon plays Karen as an obnoxious high brow know it all who constantly cheats on Ted. Somehow, Ted is the only one who can't see that Karen is the worst, making her cameo all the funnier.

Prepon returns as Karen for a quick appearance in Season 5's Say Cheese, where she ruins Marshall and Lily's Paresian Vacation and solidfies herself as Ted's worst ex, including the one who left him at the altar.