The biggest mistake I see coming from the "plan it all out" crowd is to mistake a plan for a blueprint for the history they'll write at the other end.

A plan is a set of goals, and means of attaining same, which are set out in advance. This is done with some, but not perfect, understanding of what you're going to encounter on the way.

To hold to "The Plan" in the face of all facts is to suffer a serious delusion.

Instead, you set yourself a set of goals, and means of attaining same. When reality produced a different set of events, your plan enabled you to assess the situation and modify your goals, means, and plan, appropriately.

A rock-solid plan works for simple, short-term, frequently iterated projects for which there's a high degree of familiarity and a low level of variability. Your five year plan is more a voyage of expedition than a trip to the grocery store for milk, cereal, beer, and nappies.

Similarly, last summer's road trip had a few hard-set points (Bill's house on the 2nd, DC on the 3rd), but vast bits (Montana, Four Corners, Monument Valley) were more or less made up on the spot. Flats and lost exhaust hangers threw in a few curves, but no major disasters. Pad out your estimates for unexpected arrivals ;-)