Microscopic review of biopsy tissue is considered the gold standard for diagnosis of cancer and other diseases, but prior research has shown a small yet consistent rate of errors in cancer diagnosis that is attributable to misinterpretation of biopsy specimens. This study sought to quantify error rates in breast cancer diagnosis by having a broad sample of pathologists review a standardized set of biopsies whose diagnoses had been established by expert clinicians. Although biopsies with cancer were diagnosed very accurately, specimens with atypia (abnormal tissue that may be pre-cancerous) had substantial variability, with pathologists tending to overdiagnose these specimens (i.e., ascribe a diagnosis of cancer or pre-cancerous lesions when the correct diagnosis was benign). The authors caution that the specimens used in this study were intentionally chosen to be relatively difficult to interpret, and this may have resulted in overestimating the error rate. A related editorial notes that while the overall rate of diagnostic error in this study was low, misdiagnosis of atypia does have important prognostic and treatment significance for women, and therefore pathologists should systematically consult with colleagues in difficult cases, and more advanced molecular diagnostic methods should be applied in order to reduce subjectivity in biopsy interpretation.