But what about those happy accidents? If they will make your story stronger than by all means keep that stray thought in. Sometimes, our brain does things that we haven't had time to catch up with. That doesn't mean its bad...
I am a pantser, so I am always surprised by where my story takes me. Even so, things pop up from time to time that I have just heard... a stray word... a conversation... the weather... whatever? It's then up to me to decide whether that new tidbit belongs in the story.
For NaNo, however, we should leave those happy accidents in the story and edit later. Editing during NaNo can cause us not to reach our goals. However, some go crazy if they don't edit. Don't get me wrong I find myself changing words or fixing spelling all the time, even in November. Is this bad? Not in the long run, perhaps not even in the short run. So, it's up for you to decide.
The most important thing during National Writing Month is that you're writing everyday! That is the lesson we all can take away from this crazy month of mad-dash writing... WRITE EVERY DAY!