Using seeAllData=true in test classes or methods creates dependencies on existing data in the Salesforce org, which violates the
fundamental principle of test isolation.
When tests rely on existing org data, they become unreliable and unpredictable. The same test might pass in one environment but fail in another,
simply because the required data exists in one org but not the other. Even worse, tests that have been working for months can suddenly start failing
when someone deletes or modifies the records they depend on.
Test isolation ensures that each test is self-contained and creates its own mock data. This approach guarantees that tests will behave consistently
across different environments - development, staging, and production orgs.
Additionally, there’s a technical restriction in Apex: seeAllData=true cannot be used when @testSetup methods are present
in the same test class. This combination will cause a compilation error, preventing the code from being deployed.
What is the potential impact?
Tests become unreliable and may fail unexpectedly when dependent org data is modified or deleted. This can block deployments and create maintenance
overhead. Tests may also behave differently across environments, making it difficult to ensure consistent application behavior.
