Quotes: You Wouldn't Believe Me If I Told You

Ravirn: If I tell you, there's absolutely no way you're going to believe me.
Eris: I don't believe you.
Ravirn: What did I just tell you?
Eris: That I wouldn't believe you. But that's got nothing to do with my not believing you.
Ravirn: I know that, because I know why you don't believe me.
Melchior: (*facepalm*) I don't believe this.
WebMage

Rufus: You're not going to believe this.
So Random!, right before saying something that's completely ridiculous, but ultimately turns out to be true.

"You would not believe what I've been through tonight."
Mr. Potato Head, Toy Story 3