I do it. In my current writing project, which I started some months ago, I had this image in my head that at the end of ACT I, character A was going to seek out character B for some help. The twist is, character A is a professional killer, and B knows of him and is terrified of him, so A has to convince B he is not there to kill him, and actually needs his help, and so forth.
I don't write outlines, so my characters were ill defined, but the first scene I wrote was this meeting. Which helped me define the characters. I liked some of that, but even when I had fixed everything I could, it wasn't great, because I had to load too much into exposition or dialogue explanation in order to make it all work.
That scene helped me figure out what had to come first in the first Act. So I wrote that, all the way up to the meeting scene. Then 90% of that was useless, I rewrote the meeting scene, with the foundation of five scenes before it, and now it flows smooth. It isn't too loaded, and as it turns out, B is not that terrified of A after all, because A got some advice from another character earlier in ACT I, specifically because it would have been too stupid of him to approach B cold.
So the first thing I wrote, and had to get on paper, was mostly garbage, but helped me better understand who A and B are and why B would help A, and what must have happened before this. It is still the turning point scene in Act I, just not as I originally imagined it. A still has a problem, B can solve it with some risk to himself, and B (in this particular case) realizes he really wants A to succeed.