Electronic prescribing systems have been shown to prevent medication errors in the outpatient setting. However, such systems do not routinely notify pharmacies if a clinician has decided to stop prescribing a medication, creating the potential for harm. Conducted in 15 primary care practices that use a commercial electronic medical record system, this study found that 1.5% of prescriptions discontinued by physicians were subsequently dispensed at least once by pharmacies. Since these medications included high-risk therapies such as antidiabetic and antiplatelet agents, some patients may have experienced preventable harm as a result. This study identifies a previously undocumented type of error in ambulatory care and describes the need to harness technology to facilitate bidirectional communication between providers.