A famous example is that fellow, uh, uh... Well, I can't think of his name, but for years he's insisted he's a Russian prince.
9 seconds sound clip from the Miracle on 34th Street (1947) movie soundboard.
You can hear this line at 00:34:47 in the Blu-ray version of the movie.
Quote context
[...]
- He didn't answer the questions correctly?
- I don't think there's any doubt about it. He should be placed in a mental institution.
- His is a delusion for good. He only wants to be friendly and helpful.
- There's been all kinds of evidence to prove him wrong, but nothing has shaken his story.
- No.
- He owns a famous restaurant in Hollywood and is a highly respected citizen.
- I'm afraid I'll have to disagree with you again.
[...]