Yes, it has happened to me. I think of a plot, dialogue and as I'm writing the scenes in my head change lol. Pretty funny.
They are doing it with irritating regularity right now. Two stories that have gone 5000 words without ever getting to the "big scene" and taken off in directions I never intended (one might actually end up on the blue site since I'm not sure the sex I had planned even fits anymore). I've put both on the backburner and started one that is pretty much just a pure sex piece with minimal plot and it's going well.
I would not know what is normal for other people. I have basically never had a planned outline for a story. I have an idea. I start writing and eventually a story will emerge. At times I see a picture that I use to illustrate the story which actually gives me the story to write. The only story where I used what some might call an outline was for my steampunk mystery called Murder on the River Thames. it was so intricate in its details that I had to write down tons of research and a sort of idea about what would happen. But it still wrote itself in many ways.