Increasing numbers of hospitals are implementing bar-coding systems to prevent errors in patient identification. In the present case, a diabetic patient admitted to a teaching hospital was mistakenly given the bar-coded identification wristband of another patient who was admitted at the same time. When a laboratory result that documented the diabetic patient's severe hyperglycemia was entered into the other patient's electronic medical record, the latter patient seemed to have a very high glucose level and was almost given what could have been a fatal dose of insulin. This near miss shows that computer systems, although having the potential to improve safety, may create new kinds of errors if not accompanied by well-designed, well-implemented cross-check processes and a culture of safety. Moreover, computer systems may have the pernicious effect of weakening human vigilance, removing an important safety protection. Researchers should continue to study real-world implementation of computerized systems to understand their benefits and potential harms, and administrators and providers should seek ways to anticipate these harms and mitigate them.