if your frito pie hasn't been cooked enough to congeal in the vague liquids secreted from the ingredients, it's not real frito pie. it's just a taco in a bag.
I read a story in which the author, whose hero was Texas, mentioned someone taking a "slice" of Frito pie, and thus exposed himself as a poseur.
In a different story, by a different author, the hero went to Waffle House and bought a Belgian waffle - making it clear he had never in fact visited a Waffle House.
In the Walking Dead, they go from Florida to South Carolina and can't find a single firearm in any house. "Huh" I thought. "The screenwriters are from Los Angeles."
What examples can you give of a writer unintentionally giving away his lack of knowledge?