Incorrect Access Control occurs because the application processes the `X-HTTP-Method-Override` header in the `ServeMux.ServeHTTP()` function without restricting allowed methods. When a POST request with `Content-Type` set to `application/x-www-form-urlencoded` includes this header, the request method is rewritten to an arbitrary value provided by the attacker before routing. This behavior enables the bypass of method-based access controls enforced by Web Application Firewalls (WAFs) or upstream proxies.