In the book Blood Memory the author talks about a bridge over the Luapula River by the Mambilima Falls on the border of Congo and Zambia, but the bridge was never actually completed and still remains uncompleted today
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?