Ever have a fic switch genres on you?

The story I'm currently working on as a somewhat fantastical premise: Guy makes a wish and chooses to completely forget his former best friend turned bitter enemy. This in turn, confuses everyone and upsets the enemy more than he ever would have imagined.

The premise and initial vibe of the story was more comedic, with the enemy trying various tactics to either prove the guy is faking or jog his memory. And yet, the more I've written it, the more dramatic it's become as the enemy starts coming to terms with how he went about betraying his friend. And in turn, the guy who's forgotten everything is now undergoing an additional battery of tests, because .... well, most folks don't just completely forget a dude.

Has anyone else ever had the tone shift on them so drastically, mid-story?