This paper describes the use of Obstacle Analysis to identify anomaly-handling requirements for a safetycritical, autonomous system. The software requirements for the system evolved during operations due to an on-going effort to increase the autonomous system?s robustness. The resulting increase in autonomy also increased system complexity. This investigation used Obstacle Analysis to identify and to reason incrementally about new requirements for handling failures and other anomalous events. Results reported in the paper show that Obstacle Analysis complemented standard safety-analysis techniques in identifying undesirable behaviors and ways to resolve them. The step-by-step use of Obstacle Analysis identified potential side effects and missing monitoring and control requirements. Adding an Availability Indicator and feature-interaction patterns proved useful for the analysis of obstacle resolutions. The paper discusses the consequences of these results in terms of the adoption of Obstacle Analysis to analyze anomaly-handling requirements in evolving systems. Keywords: contingency requirements, obstacle analysis, safety-critical software, requirements evolution, autonomy, anomaly-handling.