There's several risks that need to addressed when planning for retirement. One of the risks is assuming that if you earn a particular average rate of return on your investments in retirement that you will be ok. However, the order of the rates of return and the range of the returns can have a significant impact. The difference between average rate of return and the real rate of return is not often understood.
In this video I explain this simply, and even show how earning a lower average rate of return might mean more money could be available for you to spend in retirement. It all depends on the order and range of the returns that you earn that you achieve over time.