What kind of errors might I encounter when using a Google Indexing API key, and how do I troubleshoot them?

Common errors include invalid API key, insufficient permissions, exceeding quota limits, and malformed requests. To troubleshoot, check your key's validity, verify the service account has the correct permissions, monitor your API usage in the Google Cloud Console to ensure you're within quota, and validate your request payloads against the API documentation. Review error messages carefully for specific clues.