Challenging your assumptions: finding the fatal flaw

For many years, I have talked about the importance of incorporating the practice of challenging outcome-determining assumptions into strategic planning. Unchallenged assumptions are often corporate blind spots, and inherent in blind spots are the difficulty in finding them.
Your business plan is based on a bunch of commonly agree-upon thoughts about how your operating environment is going to evolve. Inevitably, the accuracy of these assumptions change over time, hence the criticality of systematically assessing them to make sure your next business plan is not based on outdated beliefs.
This is a great article that explains this concept, within the context of the sudden shift in oil prices.