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

What common mistake do developers make with Headers and cookies, why does the mistake cause problems, and what is the safer approach? One recurring problem with headers and cookies is applying it without understanding the boundary it protects. Always returning status 200 for errors instead of using HTTP status codes that describe the result of the request. In addition, Responses can carry headers and cookies when an application needs to control caching, content type or client state. 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.