Collision against conditions

I have a "fireball" object. The object is instantiated at the position of the hero, and then is moved using AddForce().

The "fireball" is also given 20 health, and whenever it hits something it loses 10 health.

The code I'm using (rather, just the conditions) should work, though it's not. Does something special need to be done when you handle colliders?

     void OnCollisionEnter()
     {
         if (collider.gameObject.tag == "Cube" || collider.gameObject.tag == "Terrain")
         {
             collider.gameObject.SendMessage ("ApplyDamage", 10);
             Debug.Log (collider.gameObject.GetComponent<Spells>().S_CurrentHealth);
             if (this.gameObject.GetComponent<Spells>().S_CurrentHealth <= 0)
             {
                 Destroy(this.gameObject);
             }
         }
         else 
         {
             Debug.Log ("Nothing");
         }
         
     }

What this is SUPPOSED TO DO is apply damage to the "Cube" hero (the point of the fireball's origin) and also apply damage to the terrain object. (I was hoping to reach a null-reference-error, since I don't have an "ApplyDamage" function attached to the terrain.

Instead what's happening is the Debug.Log is telling me that there's "Nothing."

The funny thing is though when I switch the conditions around to

`if (collider.gameObject.tag != "Cube" && collider.gameObject.tag != "Terrain")`

The script applies damage twice; once when it is instantiated and another time when it hits the terrain, basically saying that it's colliding with the two things that it's not supposed to collide with (Opposite day, by chance?)

Last point of clarification

For testing purposes, I used GetComponent directly instead of applying it to a variable first.

So yeah, does anyone know why this could be happening?

UPDATE

Whether or not I switch the "Or" and "And" operators, it does the same thing.