Diagnostic error underlies about 10% of adverse events occurring in hospital practice. However, there have been very few studies considering means of improving the mechanisms of diagnosis. As a result, misdiagnosis has been described as 'the next frontier for patient safety'. In this study of case records of patients admitted to hospital as emergencies, some key factors that may underlie diagnostic errors were assessed. From these observations, possibilities for improving the quality of diagnosis and the planning of subsequent care are explored. This paper shows that cognitive biases, believed to distort diagnostic conclusions, can be applied quite specifically to stages in clinical care. These observations led to the proposal of a clinical assessment with a method designed to encourage analytical reasoning. In addition, minor defects in standard practice are shown to adversely influence diagnosis. The findings of this study offer possible means of improving the quality of diagnosis and subsequent patient care, and perhaps pave the way for prospective studies.