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From an interview with Karl Weick (http://www.wiredcottages.com/managementthinkers/interviewkarleweick.htm), published in the HBR:

You've often said that plans are overrated, that they can actually make things worse for organizations.

Yes, I usually urge executives to fight their tendency to want to plan everything. Most plans are too specific, and the details create the illusion that the plan grasps everything that is going on and therefore can be trusted. As a result, when you have a plan, you tend not to look for things that disconfirm it. Plans are the opposite of gossip in that they lure us into the trap of overlooking the unexpected. They also deceive us into thinking that we know more than we do. The worst aspect of plans is that they heighten the tendency to postpone action when something unexpected happens. People do nothing while they stand around asking themselves, "What was I supposed to do in this kind of emergency?