Some time ago, I used a list of highways to complain about the structure of questions on the RUP test. If this wikipedia article is accurate, my multiple choice question was even harder to parse. It turns out that US 50 may actually be an interstate, though it is not marked as one.
I arrived at this via a route that started at this interesting blog about weird maps. (hah! I said "arrived"! and "via"! I'm so dang funny. Oh, and "route"! Snicker.)