The implementation of new computerized provider order entry (CPOE) systems can have unintended consequences, including adverse clinical outcomes. As CPOE systems evolve, health systems are beginning to transition from older versions (with only limited decision support capabilities) to more sophisticated systems. This analysis of a primary care practice that underwent such a transition found fewer overall prescribing errors with a newer system. However, this improvement was largely due to the new system's ability to prevent "do-not-use" abbreviations in prescriptions; the incidence of other types of prescribing errors actually increased for the first 3 months after implementation. Even with experienced CPOE users, novel systems evidently have the potential to adversely affect patient safety.