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

What common mistake do developers make with Interfaces, why does the mistake cause problems, and what is the safer approach? One recurring problem with interfaces 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, Interfaces let application code depend on a contract while the container chooses a concrete implementation. 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.