What should come into your business plan is how you assess it, how you foresee anything occurring that could have an adverse impact and how you would deal with it in the right ways. If you are looking to obtain funding from a bank or people you know, it is essential to show what the risk factors are in the proposed business and how you plan to defend against them.
Once you have a plan in place, it's important that you follow it. If you're achieving your goals you should stick with the plan. If you are not achieving your goals then you will have go back, analyze your plan to find out what is working, what is not working and why it is not working. A plan is not etched in stone. It is subject to change. As time goes on, things change in this world and businesses like everyone else are subject to change. A good plan will reflect changes that a company has to make to keep it competitive and successful.