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

What common mistake do developers make with Cookies, why does the mistake cause problems, and what is the safer approach? One recurring problem with cookies is applying it without understanding the boundary it protects. Storing large datasets or secrets in cookies when server-side session storage or another secure persistence mechanism is more appropriate. In addition, Cookies can store small client-readable or server-managed values according to the application's needs and security policy. 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.