When something is going rather badly, or I know I just wrote a crap line, I put a bold X right there in the text and move on. I've gone back and edited a line here or there, but usually only in dialogue so that when the characters decide to talk about something else, it makes sense.
I also grabbed a notebook and opened it up so I have two side-by-side sheets. On one side, I wrote a basic outline (only about a 1/8, because that's all I know). On the other open-faced page, I drew little arrows like "X needs to find this out before this" or "include a piece about this." For example, I forgot to write in one place where I show that one character has a gun (duh!).
So it looks something like this:
1. Blah blah <--gun needs to go here
2. Stuff happens <--need more foreshadowing
3. Lookit! Conflict w/Gun!
The two sheets just ensures I have a lot of extra writing room so one page doesn't get too cluttered (which in undoubtedly will). Since I made a note of the missing and/or bad elements, I treat the rest of the story like it's already in the scene and keep going. It can always be fixed later.
All in all, this Nano is going to end up more like a big outline than anything. It's going to need a lot of extra work when it's finished.