Plan that your plan will not work

"Everyone has a plan until they get punched in the mouth." I love this insightful quip from Mike Tyson. 

There is also the variation that may sound loftier, "Man plans, and God laughs". 

You should always plan that you plan is usually not going to go as planned. The foundation of risk management is the appreciation that what you may think will happen with your portfolio will not happen. Most engineering problems plan for any contingency and then assume that the design will work. There are stress and calculations that can be made, but the solution is scientific. The behavioral aspects of finance, people make mistakes, creates a clear situation that plans will fail.