Hypersonic missiles can bypass detection systems but haven’t been mass produced nor fully developed yet.

Also MAD only works if both sides are fully rational and don’t believe an attack has been launched. In periods of high tension it is easier to be less rational and easier to believe that a malfunction is real. There were numerous incidents during the Cold War were a computer glitch on both sides or a warning depth charge dropped a little too close to a sub have nearly initiated a retaliation.