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. Trusting raw input and using it directly in database updates or application decisions without validation and authorization. In addition, Request objects expose HTTP headers and cookies through framework-friendly methods. 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.