title: Error Attaching Unity Script to gameObject
body: <p>I got an error when I try to attach my script to a gameObject. </p>
  
  <blockquote>
    <p>'EnemyBehaviour' is missing the class attribute
    'ExtensionOfNativeClass'!</p>
  </blockquote>
  
  <p>Another times, I got:</p>
  
  <blockquote>
    <p>Can't add script behaviour CallbackExecutor. The script needs to
    derive from Monobehaviour!</p>
  </blockquote>
  
  <p>I have made sure my class derives from MonoBehaviour.</p>