Otherwise you plot yourself into a corner, like I've just done, and get completely stuck.
I mean, I can see this being a useful exercise, at the very least, and I can see me using some of these characters and ideas. But right now, I just can't see myself actually finishing this thing. Okay, so it doesn't have to make sense; that's part of NaNoWriMo. But try telling my writer/muse/whatever that. If I can't see anywhere for the plot to go, and I can't even see any way of fixing the existing stuff I've written, I don't seem to be able to get on with the next scene. I've tried a few things; I've even tried starting from the end and working backwards. But it doesn't seem to be working.