To reproduce:
1. Open the attached project
2. Attach the script to the Main Camera
3. Run the scene
4. Observe the logs in the console
5. Observe the script content

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Script tries to assign a collider that does not exist to a variable then add it to List<Component> if it's not null. The variable gets added to the list but components[0] is null:

var obj = GetComponent<Collider>(); //Get a component that does not exist
components.AddNotNull(obj); //Add it if it is not null

//Check result
Debug.Log("Number of items: " + components.Count + ".");
Debug.Log("First value: " + components[0] + ".");

Explanation:
http://blogs.unity3d.com/2014/05/16/custom-operator-should-we-keep-it/