Candyland, a small chain of candy stores in Minnesota, holds the federal trademark registration for CHICAGO MIX, which it uses for its mixture of salted, caramel, and cheese-flavored popcorn. (The trademark registration issued in 1992 and states a first use date of 1990, although Candyland's owner claims the company has used it since 1988. Interestingly, Candyland does not appear to claim any trademark rights in CANDYLAND, a mark which happens to be owned by Hasbro, Inc.)
Unfortunately for the Minnesota Candyland, many other companies also use CHICAGO MIX or related terms to describe their similar products, including Chicago-based Garrett Popcorn Shops, which claims to have been selling CHICAGO MIX popcorn "for decades" (and therefore, presumably, before Candyland acquired its trademark rights). (Garrett's CHICAGO MIX omits the salted popcorn, combining only caramel and cheese-flavored popcorn. It would be interesting to compare them side-by-side, but Garrett's popcorn is best eaten fresh from the store, and I imagine the same is true for Candyland's.)
Ironically, Candyland's own website contains a long list of competitors that use CHICAGO MIX or related terms. It's easy to find many more that aren't on Candyland's list (e.g., Utz Chicago-Style, Nuts on Clark Chicago Mix, Popcornopolis Chicago Style, Brooklyn Popcorn Chicago Mix, Popcorn Haven Chicago Style, The Popcorn Fanatic Chicago Mix, Popcorn Palace Chicago Chic, Great American Popcorn Chicago Mix, Popilicious Chicago Mix, Pittsburgh Popcorn Chicago Blend, Kettle Boys Chicago Mix, and Chicago Gold Popcorn Windy City Mix). It seems to me that CHICAGO MIX is likely a generic term for a mixture that includes caramel and cheese-flavored popcorn. The CHICAGO MIX term is widely used around the country, but I wonder how many people outside Minnesota have ever heard of Candyland.
Candyland recently filed lawsuits against Garrett's and two other companies (Snyder's-Lance and Cornfields) for infringing upon its CHICAGO MIX trademark. (One of the allegations in the lawsuit is that Garrett's use of the CHICAGO MIX mark is likely to deceive consumers as to the origin of Garrett's popcorn, apparently making them think that CHICAGO MIX popcorn comes from Chicago rather than Minnesota.)