In April 1963, the Soviet nuclear submarine K-33 collided with the Finnish merchant vessel MS Finnclipper in a mist in the Kattegat strait off the North Sea.

With engine noise on the port side of the Finnclipper, the Russian submarine suddenly emerged from the mist. The Finnclipper veered sharply to starboard but was unable to avoid a collision.

Without revealing their identity, officers on board the submarine told the Finnish captain that the submarine’s side had suffered severe structural damage. The Finns, however, read the number 921 clearly on the side of the submarine, identifying her as the Soviet nuclear submarine K-33.

The severely damaged K-33 limped to Murmansk for repairs.

Source: Wikipedia