This experience report describes the lessons learned from the use of Software Failure Modes and Effects Analysis (SFMEA) for requirements analysis of critical spacecraft software. The SFMEA process was found to be successful in identifying some ambiguous, inconsistent, and missing requirements. More importantly, the SFMEA process, followed by a backward analysis somewhat similar to Fault Tree Analysis (FTA), identified four significant, unresolved requirements issues. These issues involved complex system interfaces and unanticipated dependencies. Our results challenge some current views on the limitations of SFMEA and suggest that recent efforts by researchers to integrate SFMEA with a broader FTA approach have merit.