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

What common mistake do developers make with Flash data, why does the mistake cause problems, and what is the safer approach? One recurring problem with flash data 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, Flash session data is intended for short-lived messages such as success or error notices after redirects. 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.