Okay, first of all, I am a "beginning to end" writer. I know there are a fair number of people out there who write scenes in whatever order they come, and then put everything together at the end. And that's a perfectly valid way to write a novel. My friend Robin D. Owens writes that way.
I can't, because my scenes don't come in "whatever" order. This happens first. And because that happened, then something else happens and the characters react this way--everything builds on what came before. However, sometimes I do realize while I'm writing away, that I should have written a scene that happened earlier. And most of the time, I will stop and go write that scene before I go on, because I can't really go on until I know what's going on in that scene.
In the Victorian steampunk book I'm currently working on, I realized somewhere in Chapter 5 that I really ought to write a new series of scenes about what's happening in Paris while my hero and heroine are busy in Transylvania. So, twice, I had to pause in the middle of the action and go back and write a scene that I'd forgotten to write when I came to the proper place for it, and write about Paris.
Today (and I'm really proud of myself for it), I switched locales to Paris right when I needed to switch. Mostly it was because things were getting dicey with my hero, and I had no clue what would happen next. Or, I knew what needed to happen, but I wasn't exactly sure how my heroine was going to pull it off. So, I went to Paris and wrote about that. It worked. I kept writing even though I didn't know where I was going.
Of course, I still have to figure out what my heroine is going to do, but... hopefully, when I sit down and write, it will come.
8 pages today, so far.