Background: The risk of local recurrence (LR) after soft tissue sarcoma (STS) resection is higher in the setting of inadvertent positive margins (IPMs). This study assessed whether both tumor- and surgery-related factors contribute to IPMs, and whether tumor- versus surgery-related IPMs differ in LR or overall survival (OS). Methods: Retrospective review of a tertiary center database identified patients with IPMs following STS resection between 1989 and 2014. Of 2234 resected STSs, 309 (13%) had positive margins; 89 (4%) were IPMs. Mean follow-up was 52 months, mean tumor size was 9.2 cm, and 55% were high grade. Cases were categorized as surgery-related (67, 75%) or tumor-related (22, 25%). Results: There was a significant difference in positive margin location, with the deep margin commonly involved in surgery-related IPMs (55% vs. 9%; p < 0.001). Tissue type also differed (p = 0.01), with surgery-related IPMs frequently in muscle (33%), while tumor-related IPMs favored subcutaneous tissues (41%). STSs with surgery-related IPMs were larger (p = 0.01). Histologic subtypes differed (p = 0.02), with myxofibrosarcoma and undifferentiated pleomorphic sarcoma/malignant fibrous histiocytoma predominating in tumor-related IPMs (82%). The cumulative probability of LR after IPMs, with death as a competing risk, was 28% (95% confidence interval [CI] 18–35) at 5 years and 37% (95% CI 24–45) at 10 years. Mortality was 28% (95% CI 18–38) at 5 years and 38% (26–50) at 10 years. There was no difference in LR (p = 0.91) or OS (p = 0.44) between surgery- and tumor-related IPMS. Conclusions: IPMs after STS resection results in substantial LR risk. While demonstrating distinct surgery- and tumor-related contributions, there was no between-group difference in LR or OS. These results may aid in avoiding IPMs. Level of Evidence: Therapeutic Level III, retrospective comparative study.