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Trying to use the result of a getter that's returning null:

Note: Be carefull, GameObject.Find, GameObject.FindWithTag, GameObject.FindObjectOfType only return gameObjects that are enabled in the hierarchy when the function is called.

Trying to use the result of a getter that's returning null:

Retrieving a component that isn't attached to the GameObject and then, trying to manipulating it:

private void Awake ()
{
    // Here, you try to get the Collider component attached to your gameobject
    Collider collider = gameObject.GetComponent<Collider>();

    // But, if you haven't any collider attached to your gameobject,
    // GetComponent won't find it and will return null, and you will get the exception.
    collider.enabled = false ;
}

Accessing an element of a non-initialized array

private GameObject[] myObjects ; // Uninitialized array

private void Start()
{
    for( int i = 0 ; i < myObjects.Length ; ++i )
        Debug.Log( myObjects[i].name ) ;
}

Accessing a GameObject that doesnt exist:

Retrieving a component that isn't attached to the GameObject and then, trying to manipulate it:

private void Awake ()
{
    // Here, you try to get the Collider component attached to your gameobject
    Collider collider = gameObject.GetComponent<Collider>();

    // But, if you haven't any collider attached to your gameobject,
    // GetComponent won't find it and will return null, and you will get the exception.
    collider.enabled = false ;
}

Accessing a GameObject that doesn't exist:

If you try to get a component that isn't there and then, try to access it:

Accessing a GameObject that doesnt exist:

Trying to use the result of a getter that's returning null:

Less common, but annoying if you don't know it about C# delegates:

Manipulating a GameObject / Component that has not been specified in the inspector

Retrieving a component that isn't attached to the GameObject and then, trying to manipulating it:

Accessing an element of a non-initialized array

private GameObject[] myObjects ; // Uninitialized array

private void Start()
{
    for( int i = 0 ; i < myObjects.Length ; ++i )
        Debug.Log( myObjects[i].name ) ;
}

Accessing a GameObject that doesnt exist:

Trying to use the result of a getter that's returning null:

Accessing an element of a non-initialized array

private GameObject[] myObjects ; // Uninitialized array

private void Start()
{
    for( int i = 0 ; i < myObjects.Length ; ++i )
        Debug.Log( myObjects[i].name ) ;
}

Less common, but annoying if you don't know it about C# delegates: