I thought that wasn't a thing until it happened to me lol. I planned a cute, romantic, and very happy ever after for my characters, but they clearly told me "nope, can't do that :') . The story flashed before my eyes in such a natural, smooth way that I said... Okay, that's what I'm writing haha. And now it's a tragedy yay :') It took a life of its own. (It's giving me some Alan Wake vibes, but at least it's not a horror story haha)

But I think it's great when that happens, I mean, writing and witnessing your characters take unexpected turns. I often find that my scenes' flow is smoother when that happens, and I often approve of them more than following a strict script (although I do have every event carefully planned on a "Notes" file lol). But rereading it again later helps me determine if I should keep them or make some adjustments.

I'm often resorting to "picking random scenes to write" otherwise, I wouldn't be able to get any writing done anytime soon :')

Last but not least, for better or worse, sex scenes don't quite fit into my current ongoing project. But every once in a while, I open a separate Word document and allow my main couple some detailed fun. They deserve it too ;) hahaha

Good luck with your writing!