Other mistake: Just after the Royal Navy destroyer has sailed through and depth charged the area where the British survivors are, there is a shot of the churned up sea with sombre music playing, and the seagulls are flying backwards.
Factual error: The footage of the Murmansk convoy firing at attacking German planes is actually Pacific footage of Kamikaze attacks. The density of the fire was only possible because of the lavish AA available then, unlike the meager AA of convoy escorts.
Revealing mistake: In various scenes before the Compass Rose is supposed to be fitted with radar, it's obvious the 'lantern' round the already fitted radar aerial has been removed and the aerial itself covered with a tarpaulin to simulate the ship in its earlier, pre-radar, state.
Continuity mistake: In a scene supposed to be after the ship has been fitted with radar, it is shown entering harbour with the 'lantern' removed and the aerial covered, which is how it's shown before the ship has the radar fitted.