Eating secret stuff

In-n-out is a famous burger chain in the United States, and takes pride in proposing a very simple menu (burger-fries-drink). It is not as big a chain as, say, McDonald's, but Yours Truly has come to like those cheap and simple burgers (especially their sauce). McDonald's and others have gone down as my age went up (let's not talk senility today).
Badmouth gives us all the nitty-gritty details about a secret menu that In-n-out would have available...