The Capability Maturity Model of the Software Engineering Institute (described by Watts S. Humphrey in Managing the Software Process, by Addison-Wesley, 1989) defines software fault prevention as a key process area required at Level 5 — the “optimizing” level. We conducted a detailed study on a number of software faults found during low-level design, coding, and unit testing in the 5ESS®-2000 switch software development environment. We discovered that the faults found during those phases were typically limited in their scopes of impact, but they usually account for more than 60% of the total software faults found in a software product before its delivery. We also found that the corrective and preventive tasks of each fault identified during those phases were very likely manageable by one development engineer. The results of the study of frequent software faults served as a base of knowledge that enabled engineers to strengthen their design and programming capabilities and prevent faults from occurring again.