There are three interdependent factors that drive our software development processes: interval, quality and cost. As market pressures continue to demand new features ever more rapidly, the challenge is to meet those demands while increasing, or at least not sacrificing, quality. One advantage of defect prevention as an upstream quality improvement practice is the beneficial effect it can have on interval: higher quality early in the process results in fewer defects to be found and repaired in the later parts of the process, thus causing an indirect interval reduction. We report a retrospective root cause defect analysis study of the defect Modification Requests (MRs) discovered while building, testing, and deploying a release of a transmission network element product. We subsequently introduced this analysis methodology into new development projects as an in-process measurement collection requirement for each major defect MR. We present the experimental design of our case study discussing the novel approach we have taken to defect and root cause classification and the mechanisms we have used for randomly selecting the MRs to analyze and collecting the analyses via a web interface. We then present the results of our analyses of the MRs and describe the defects and root causes that we found, and delineate the countermeasures created to either prevent those defects and their root causes or detect them at the earliest possible point in the development process. We conclude with lessons learned from the case study and resulting ongoing improvement activities.