Amended pathology reports produce rework, confusion, and distrust. To develop a reproducible amendment taxonomy we derived a classification from 141 amended reports, then validated it with 130 new cases before 4 observers independently reviewed 430 cases measuring agreement (k). Next, agreement in classifying 30 other amended reports in 7 institutions was measured. We further tracked amendment rates, defect categories, defect discoverers, and discovery mechanisms. In the 430-case validation set agreement was excellent (k = 0.8780 [range, 0.8416-0.9144]). Among the 7 institutions, agreement was good (k = 0.6235 [range, 0.3105-0.8975]). Amendment rates ranged from 2.6 to 4.8 per 1,000 reports. Misinterpretation fractions varied least (23%-29%). Misidentification fractions ranged more widely (20%-38%). Specimen defects were least frequent (4%-10%) and report defects most frequent (29%-48%). Misidentifications and report defects inversely correlated. Pathologists discovered most misinterpretations, and clinicians found most misidentifications. Conference review revealed 40% to 80% of misinterpretations. This taxonomy produced excellent reproducibility and good agreement across institutions.