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I think one of the most common errors is spending a lot of time and effort trying to come up with future projections that are really based on unknown variables, then treating the results as facts when it is really little more than a guess. It’s like anything else, once you get intellectual “lock-in,” it is very often difficult for people to step back and reassess it objectively and logically. A non-IT example would be people who deny climate change - they just can’t separate how they feel from facts. I see it in future growth projections for businesses as well. I was having a conversation with someone starting a webzine. They had little to no name recognition, very little funding, and a modicum of talent. Nevertheless, they were planning on going from zero to a million unique visitors per month in one year. I know this isn’t networking, but it’s the same idea - planning based on unrealistic projections is not really planning.
There is a good overview on this topic at Microsoft’s Technet library that lays out some of the planning and pitfalls of network capacity planning - http://technet.microsoft.com/en-us/library/cc767907.aspx