What is a common reason for an NPSP upgrade failing in Production when it succeeded in the sandbox?

One common reason for an NPSP upgrade failing in Production, even after a successful run in the sandbox, is the lack of adequate test code coverage. In Salesforce, the platform enforces a requirement for a minimum level of code coverage during deployment to ensure that the functionality being promoted is stable and doesn’t break the system. If the coverage is inadequate, even minor changes within the NPSP or custom code not meeting coding standards can lead to issues during the upgrade process. This risk is typically less in a sandbox environment where the controlled conditions may allow for tests that aren't fully reflective of all possible scenarios in production.

The environment differences between sandbox and production can bring out issues that were not apparent during sandbox testing. When adequate test coverage is not maintained, the changes made during an upgrade can result in conflicts or errors that prevent successful deployment in production, despite having run without issue in the sandbox where conditions might be more lenient. Ensuring robust test coverage is crucial for smooth upgrades.