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

What common mistake do developers make with Testability, why does the mistake cause problems, and what is the safer approach? One recurring problem with testability is applying it without understanding the boundary it protects. Resolving every dependency manually with new inside business classes, which makes configuration and testing harder. In addition, Dependency injection makes it easier to substitute fakes or mocks during automated tests. 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.