The purpose of this study is to examine how everyday use of the Computerised Physician Order Entry (CPOE) system in the Capital Region of Denmark has led to medication errors. The study is based on clinicians' reporting of patient safety incidents. It was found that the immediate causes of the patient safety incidents primarily relates to a) a mismatch between clinical work routines and the structure of the CPOE system, b) the complexity of the user interface, and c) lack of barriers against commonly occurring, severe errors in some areas of the CPOE system. The following was concluded: A well designed CPOE system should be intuitive, provide barriers against serious mistakes, and make the correct choice an easy one. Furthermore it was concluded that it is important that the CPOE system closely supports accepted clinical work routines and that risk assessment is performed prior to implementing new design or functionality.