Generally, the process of estimating implies a significant level of uncertainty, often necessitating (informed) guesswork. So, to minimize errors with project cost estimation, a sensible aim would be to minimize the degrees of uncertainty and variability experienced by the project, which is easier said than done!
Typically, one of the biggest contributions to the level of project cost uncertainty is the human element e.g. end-users not clearly communicating their requirements resulting in a later re-definition of project scope (and increased cost).
What other areas of project cost uncertainty are you aware of, or familiar with? Do you have any suggestions how these could be best handled, when generating cost estimates?
This solution describes the biggest problem in developing accurate cost estimates in projects.