Seldom
I assume, since you are so keen to criticise, you have a solution to the problem of avoiding such a possiblity of collision between Boats?
I would not normally defend fisheads, but the submariners are probably the most professional of all the military arms. I very much doubt an error was made.
I cannot think of any solution to the problem of avoiding the risk of collisions between our incredibly quiet stealth bomber boats which must never have their positions known to anyone outside the Captain and Navigator because that would negate the entire purpose of a sub launched deterrent, and the french incredibly quiet stealth bomber boats which must never have their positions known to anyone outside the Captain and Navigator because that would negate the entire purpose of a sub launched deterrent.
One could argue that they were both doing their job to perfection, but eventually after 40 or so years, probability caught up with them.
Eventually two of the 20 or so bombers eternally bimbling around under the oceans were bound to meet, in the same way that if there were 20 B2s from a variety of nations flying around the world permanently, they would eventually have a collision.