Although computerized provider order entry has been found to prevent some medication errors, simulation studies have also demonstrated that electronic prescribing platforms can introduce or fail to prevent medication errors. This retrospective electronic health record analysis examined medication orders that were canceled. Weekend and overnight orders were less likely to be voided than weekday or daytime orders. Pharmacist, nurse, and student orders were more likely to be canceled than physician orders. Comparing the clinician-provided reason for voiding an order with the more comprehensive information in the medical record, physicians found that clinicians' reported reasons for voiding orders were largely inaccurate. The authors suggest there is unrealized potential to characterize medication ordering errors using voided-order data.