Feature flags facilitate feature parity testing by allowing real-time comparisons between old and new systems, which aids in identifying hidden errors during system migrations and ensures a smooth transition with minimal risk. This testing method, also known as TAP compare testing, operates by mirroring traffic to both systems and comparing results, capturing discrepancies in actual traffic rather than relying on engineered test scenarios. To implement feature parity testing, it's crucial to have identical interfaces on both systems, which can be achieved by creating a matching interface or adding a façade. Feature management tools, such as Split by Harness, enable multiple traffic routing options like NEW, OLD, MIRROR, and PARITY, each serving different testing purposes. These feature flags are gradually rolled out to manage traffic and ensure infrastructure can handle the load before fully transitioning to the new system. This strategy is not only applicable to service migrations but also to data store transitions, where it helps verify successful data migration and system functionality. Despite the challenges, feature flags make parity testing more manageable, providing a critical security layer that can prevent overlooked features from impacting users, thereby enhancing the reliability of system upgrades.