Computerized provider order entry (CPOE) systems prevent prescribing errors by warning clinicians about medication interactions or contraindications. However, extensive research has shown that clinicians ignore many warnings, especially those perceived as clinically inconsequential. In this randomized trial, investigators created a "hard stop" warning that essentially prevented co-prescribing of warfarin and trimethoprim-sulfamethoxazole (a combination that exposes patients to severe bleeding risks). Although the hard stop was much more successful than a less stringent warning at preventing co-prescribing, the trial was stopped and the warning abandoned because several patients experienced delays in needed treatment with one of the drugs. The accompanying editorial by Dr. David Bates points out that this study vividly illustrates the unintended consequences of CPOE, a persistent issue that has slowed the pace of CPOE implementation.