Much of the information on which deployment design is based, such as quality of service requirements and usage analysis, is not empirical data but data based on estimates and projections ultimately derived from business analyses. These projections could be inaccurate for may reasons, including unforeseen circumstances in the business climate, faulty methods of gathering data, or simply human error. Before completing a deployment design, revisit the analyses upon which your design is based and make sure your design accounts for any reasonable deviations from the estimates or projections.
For example, if the usage analysis underestimates the actual usage of the system, you run the risk of building a system that cannot cope with the amount of traffic it encounters. A design that under performs will surely be considered a failure.
On the other hand, if you build a system that is several orders more powerful than required, you divert resources that could be used elsewhere. The key is to include a margin of safety above the requirements, but to avoid extravagant use of resources.
Extravagant use of resources results in a failure of the design because underutilized resources could have been applied to other areas. Additionally, extravagant solutions might be perceived by stakeholders as not fulfilling contracts in good faith.