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

What common mistake do developers make with Security boundary, why does the mistake cause problems, and what is the safer approach? One recurring problem with security boundary is applying it without understanding the boundary it protects. Passing the entire request object to Model::create() and relying on the database schema as the only security barrier. In addition, Mass assignment protection reduces the risk of a request setting attributes such as role, status or ownership fields accidentally. 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.