The Grim Adventures of Billy & Mandy: Compulsive Lying
Grim's new girlfriend doesn't believe Grim when he claims he can tell when people are "faking it" so she fakes having a heart attack. This completely convinces everyone else, including Grim. Compulsive liars can convince others of their lies because they convince themselves so much that they act out their lies authentically. They've been known to convince themselves so absolutely, they pass phonograph lie detector tests.