Stories where the genre shifts in the story, not slowly but a hard shift. Usually in the middle or end of the story. Can sometimes revert back to the original genre.
Example: a comedy shifting to grimdark, slice of life to epic adventure and such
Examples in fiction: Madoka Magica, School Days
Read TV tropes for more information: https://tvtropes.org/pmwiki/pmwiki.php/Main/GenreShift