Occult specimen provenance complications (SPCs), which occur when there is an absence of any direct or indirect indication that a specimen switch or contamination may have occurred, constitute a significant patient safety and medical-legal problem because they can lead to misdiagnosis. However, the rate at which occult SPCs occur is unknown because, by definition, this category of errors is not identified by standard laboratory practices. In this study, we evaluated a data set comprising almost 13,000 prostate biopsies that were prospectively tested for specimen provenance errors as part of routine clinical practice. The frequency of occult type 1 errors (a complete transposition between patients) and type 2 errors (contamination of the patient's tissue with 1 or more unrelated patients) was 0.26% and 0.67%, respectively; every urology practice setting and surgical pathology laboratory type with a representative sample size experienced at least 1 type 1 and 1 type 2 error during the study period. Overall, the mean frequency of SPCs across practice settings was 0.22% for type 1 errors and 1.69% for type 2 errors. The type 1 rate showed no correlation with a surgical pathology laboratory setting or urologic practice group setting; the type 2 rate correlated solely with a surgical pathology laboratory setting. The occult SPC rate in this limited data set provides an estimate of the scope of the problem of potential misdiagnosis as a result of occult specimen provenance errors in routine clinical practice.