Historians and naval commentators have long debated Mountbatten's leadership and seamanship during the May 1940 incident.

On the night of May 9–10, 1940, HMS Kelly was operating in the North Sea off the Dutch coast during the Norwegian Campaign. In heavy fog and darkness, the ship was struck amidships by a torpedo fired by the German E-boat S-31, killing 27 crew members.
Despite catastrophic flooding and a severe list, Mountbatten and a volunteer skeleton crew kept the destroyer afloat. She was towed back to Hawthorn Leslies shipyard in Hebburn over a harrowing 91-hour journey.
Where the "Broken Protocol" Claim Comes From
While an official Royal Navy archivist has not made this claim on the record, the accusation stems from critical biographies (such as those by Andrew Lownie and Richard Hough) and naval forums:
Signaling and Visibility: Critics allege that while hunting enemy vessels in the dark, Mountbatten broke standard night-action protocol by lingering in a known danger zone and flashing unnecessary, theatrical Aldis lamp signals (reportedly joking "How are the muskets? Let battle commence").
Giving Away Position: According to these accounts, the light signals and excessive speed gave the German E-boat a clear visual fix on Kelly's position, allowing S-31 to line up its torpedo shot. 
Admiralty Reaction: During the war, Churchill and the Admiralty (not knowing the story) publicly praised Mountbatten for saving the ship, using the survival of HMS Kelly (and its later sinking off Crete in 1941, which inspired the film In Which We Serve) as a major morale booster. Privately, however, several senior Royal Navy officers viewed Mountbatten as reckless, noting that Kelly suffered an unusually high number of collisions, mine strikes, and torpedo hits under his command. It was also said that he only got to be  the Captain commanding Kelly because he was Royal.