I was recently working with one of my clients who ran into just this situation. When I first read the proposal, I sensed there was something wrong with the story but really couldn't put my finger on it. This was just a partial and I knew the author always found ways to work out of the situation. When I was speaking with her editor, she too noted the situation I had seen. Indeed, it came down to this insurmountable conflict. In this case, the conflict involved a problem that the hero and heroine had with their families. But this problem was so big that there was absolutely no way the families would ever come together. A great wedding between the hero and heroine, babies, puppies, amazing wealth and happiness would never bring them together. Literally, the only way to get to that happily ever after would be if the entire two families were dead and gone, leaving only the hero and heroine.
Too often, when writers get into situations such as this, they do resort to these miraculous solutions to save the day. "Oh wow, I didn't know I had a 3rd uncle who has been following my struggles and is now coming in to give me $4 million dollars to save the day." Or, "Wow, isn't it a coincidence that when I am at the local Starbucks, worrying how I am going to save my husband who needs a bone marrow transplant, and suddenly, his long lost twin brother is in line right ahead of me."
Umm?? Not really believable.
The point is to just look at your conflict. Is this really something the characters have a chance of overcoming on their own? Is the solution really there? Don't rely on scenarios we see for sports playoffs: "If this team wins and this team loses, but our team wins the next three but has to do so by 7 points, and it is raining that day, then we make it to the playoffs."