Cocoa is a warm drink. Miss is a title for unmarried mares. These are facts that Starlight Glimmer took for granted until she wakes up one day and finds that, for some odd reason, not only is everypony in Ponyville is using "cocoa" to mean "miss" and "miss" to mean "cocoa," but nopony has any memory of ever doing anything else. Linguistic pandemonium ensues.

Semi-inspired by the Miss Pommel scandal, Andrew Clements' Frindle, and other fun things. And yes, that gate is actually made out of chocolate.