Low-grade appendiceal mucinous neoplasms may rupture and seed the peritoneum with bland neoplastic mucinous epithelium resulting, when grossly evident, in the well-known process pseudomyxoma peritonei. Appendiceal diverticula may also rupture, resulting in mucin on the appendiceal serosa, which may raise concern for an underlying appendiceal mucinous neoplasm. We report 11 cases of ruptured appendiceal diverticula that were initially either misdiagnosed as appendiceal mucinous neoplasms, raised concern for a neoplasm, or were thought to exhibit localized pseudomyxoma peritonei. Two cases showed eversion of the appendiceal lining onto the serosa; 1 showed collision between the diverticulum and endosalpingiosis, and 3 had rare nonneoplastic epithelial cells in extra-appendiceal mucin. Most cases showed mucosal hyperplasia, mild crypt disarray, and variable reactive atypia. Eight cases had mucosal neuromas or other neural changes. None of the patients progressed to pseudomyxoma peritonei during the follow-up interval (mean 23 mo). Pathologists should be aware that ruptured appendiceal diverticula may be associated with serosal mucin and even extra-appendiceal epithelium. Failure to distinguish this process from a mucosal neoplasm with rupture may result in unnecessary therapy and cause the patient undue alarm.