Functionality failures in cases where no apparent code or infrastructure changes were made can be tricky to diagnose. Common root causes include environmental drift, where configurations or dependencies in the environment subtly change over time, leading to inconsistencies. Data-related issues, such as corrupted or outdated data, can also break functionality without any direct code changes. External factors like third-party APIs, services, or network-related issues may alter their behavior, impacting the application. Timing or concurrency issues can surface unexpectedly, especially in high-load scenarios. Finally, hidden bugs or edge cases may not have been caught during testing but reveal themselves later due to changing conditions.