What common mistake do developers make with Test data isolation, why does the mistake cause problems, and what is the safer approach?

What common mistake do developers make with Test data isolation, why does the mistake cause problems, and what is the safer approach? One recurring problem with test data isolation is applying it without understanding the boundary it protects. Putting environment-specific secrets or irreversible production cleanup logic into a seeder that developers may run accidentally. In addition, Factories are commonly combined with transactional or resettable test database strategies so tests do not depend on existing data. Safer engineering approach: keep the responsibility in the correct layer, use the framework's supported API, validate and authorize inputs, avoid hidden side effects, and verify the behavior with tests.