You can never really plan when a disaster will occur, so you need to be ready for one at any time. When my school discussed disaster baskets we noted that we should use non-perishable items, but that they would still need to be changed out every now and then to always be ready.
I agree with the above post that anticipating possible problems in the event of the unexpected is a wise thing to do. In this way, you can expect certain things to occur, however, they may not occur exactly as you envisioned them. Regardless, prior anticipation assists one in putting contingency plans in place that can mitigate some of the future problems that can rear their heads.
Elements of response plans, which are more subject to uncertainty than others is not knowing precisely how many people will be affected by the expected "unexpected." One can plan for a certain amount of people, but if the unexpected takes place at a huge event, say a tragedy at a free outdoor concert or something similar, it may be difficult to know exactly how many people will show up; how many will be children, how many will be elderly, how many may have physical disabilities, and such. A planner, who is responsible for minimizing the effects of an unexpected tragic/dangerous/destructive event, could have plans in place to facilitate the demands and needs of various people - if they knew who would be attending.
Another element of a response plan that is subject to uncertainty, more so than others, is knowing what resources to have on hand in the event of an unexpected event. It is difficult to know how many personnel will be required to handle the unexpected, and what will they need? Will they need oxygen masks, bio-hazard products, will they need significant firearms, military personnel and machinery. Will an influx of doctors and nurses, and other health care professionals be required immediately on-site. The list is endless, which is why the unexpected can have significant devastating effects.
However, this answer primarily refers to disaster type unexpected events. Unexpected events could also include unexpected illness, unexpected business problems, and more.
People say to be optimistic and not to be pessimist but when you want to expect the unexpected one then you have to be both. Thinking all the possible ways when you are planning a event and question yourself "what if this happened?". I believe that one thing that differenciate people is the way they use there ability to think. Thinking two step ahead will give you to think or expect the unexpected one.