If I don't pull out, I could wind up writing a plot thread that isn't possible. This is the problem with hard physics. But, I'm assuming this same problem occurs with everyone. If you are writing a mystery that takes place in another city, you probably have to research that city a bit before writing your story. However, once in the heat of battle, when you are elbow deep in your scene, you hit some area where you just don't know, and it very well may affect your entire scene.
What do you do? Write on? Stop and research, hoping you don't lose the moment? Ugh!
Popular wisdom says to just write and don't edit while writing. You can't control what goes on the page, but you can damn sure control what stays on the page.
I've found this as the best advice. So, whenever you see a boulder in the road, just write around it and keep going. You can always come back to it after you do some research. If you just can't find the answer, maybe it's time to just re-write the entire scene.