In professional settings, it is a standard way to acknowledge errors that were meant to be removed during the editing or development process but were missed.
It can describe receiving an item that wasn't ordered, such as a free gift from a seller that a customer initially assumes was a mistake. Colloquial and Social Contexts it slipped in accidentally
In many informal or comedic settings, the phrase is often used as a double entendre or a clichéd excuse in adult-oriented jokes and memes. Historical Usage In professional settings, it is a standard way