Endometriosis is a common gynecological disorder most often involving the pelvic region. Although it is rare, endometriosis occurring outside of the peritoneal cavity most commonly occurs within scars of the abdominal wall, but it has been reported in the lungs, pleura, kidneys, brain, and the extremities. Herein, we present 2 cases of endometriosis, including 1 case of endometriosis of the wrist that clinically mimicked a soft-tissue neoplasm and 1 case of right-groin endometriosis mimicking synovial sarcoma during the initial pathological interpretation of findings on fine needle aspiration. We also report on a third patient with synovial sarcoma to demonstrate a diagnostic pitfall. To our knowledge, endometriosis within the skeletal muscle of the wrist has not been previously reported in the literature. A literature review was performed, and we discuss how this diagnostic pitfall may be avoided. We review the techniques for diagnosing synovial sarcoma and the importance of a high index of suspicion for endometriosis when investigating any soft-tissue mass in a female patient of reproductive age. Adequate pathological evaluation in conjunction with the correlating clinical and radiological information should help facilitate an accurate diagnosis.