A British ballistic missile sub (HMS Vanguard) collided with the French sub Le Triomphant out in the Atlantic over the weekend. There was no trouble and both subs are back in their respective ports.
Which points out a teeny problem. A ballistic missile submarine's best defense is to become what is known as a "hole in the water," or completely undetectable.
I guess there's a disadvantage to being so stealthy.