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

What common mistake do developers make with Response types, why does the mistake cause problems, and what is the safer approach? One recurring problem with response types is applying it without understanding the boundary it protects. Putting database queries, validation, notifications and complex business rules into one very large controller method. In addition, A controller can return views, redirects, JSON responses, downloads or other supported response objects. 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.