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A dog ate Dan's heart.
Let's just get this one out of the way right off the bat...
Episode: 618, "Searching For a Former Clarity"
What: Just when Dan was about to get a life-saving heart transplant... a stoned dog ate the organ.
Why: We hate to say it... but this could be the most memorable moment of OTH ever. Even people who have never seen an entire episode of OTH remember this scene.
Our favorite part? Lucas's face as he watches it all go down. He's got "you totally deserved for your heart to get eaten by a dog" written all over his squinty forehead.