The captain of the 'Weser' takes the 1st Lieutenant for the U-boot's captain, and addresses him accordingly as 'Kapitänleutnant'. Now the Lieutenant, in perfect dress, clearly displays the double stripes of his actual rank. It is difficult to imagine any naval officer making such a mistake. But the movie (unlike the novel) depicts the Weser's captain as some kind of an operetta officer. Even so, he is very formal, and such a blunder does not seem very compatible.
The "false alarm" crash dive is triggered by a watchman spotting a "plane". He points to the rear end of the ship, slightly left. This is starboard, not 'port' (D: "backbord"), as he tells the captain when submerged.
On the Weser, the captain says he has never had fresh figs before, but he demonstrates the correct technique of opening one to get at the flesh inside. However, throughout the movie he proves to be a wise person who can figure out fast the best way to eat the fruit.
The sawfish logo on the boat tower differs in shape between shots (full scale model leaving harbor in La Rochelle vs. smaller model during first emergency dive), most notably the mouth line of the fish is different.
During the start of the film the all footage of the submarine show that is was armed with a 8.8 cm SK C/30 naval deck gun and a 2 cm FlaK 30 AA gun on the deck. In the final footage of the submarine returning to France whilst the crew is singing it is missing both guns, presumably stock footage of another U-boat or an incorrectly made model.
When Lt. Werner (War Correspondent) is looking through pictures from one of the other officers family in the snow, you are shown the current picture he looking at. He clearly turns over to view the next picture, only to see the same picture again.
As Das Boot sets sail to La Rochelle towards the end of the film, the captain and one of its crew members are standing on the bridge of the sub. A couple of seconds later, in a more distant camera shot showing the entire sub, no one can be seen on the bridge.
After making first contact with the destroyer, while they await orders from the Kaleun, Frenssen's and Preacher's positions differ between subsequent shots. Moreover, Ullman sits where Preacher should be sitting.
In the depth charging after the torpedo run, the boat goes deeper and deeper until finally rivets start to 'pop'. In fact, the pressure hull of the Type VIIc boat was welded, not riveted. This was also one of the reasons the type was able to survive depths of 220 meters and beyond in the first place.
U-32 is stated to have made contact with an enemy convoy. Following the convoy attack, Kriechbaum reads a report from U-112. U-32, a Type VIIA boat, actually sank in October 1940, a year prior to the events of this film. U-112 was a proposed Type IXB boat, which was designed to be a seaplane carrier but was never completed.
In a rather famous sequence of the movie/TV-series, Bootsmann Lamprecht (Uwe Ochsenknecht's character) informs the crew that their football-team (Schalke 04) had just lost 5:0 and was "out of the running" (in a non-specified competition). Given the time-frame of the movie (early Oct. to 24 Dec. 1941), this is historically wrong. While the German national championship during this time used a KO-system in its final stages (unlike today), the final for the 40/41-season was held in June '41 and that of the 41/42-season in July '42. Neither date fits the time-frame of the movie and in any case, Schalke made it to both finals (losing in 40/41 and winning in 41/42) - so they never got eliminated in either season. In fact, Schalke was such a dominant team during this time, that they only lost a single match (the final) during the entire 40/41 league-championship.
The only other competition they could be talking about is the German Cup ("Tschammerpokal") of 1941, whose semi-finals and final fit the time-frame (12 Oct and 2 Nov, respectively). However: In that competition, Schalke won their semi-final (6:0) and only lost in the final (2:1 against Dresdner SC).
SS Weser was built in 1922 by Norddeutscher Lloyd and was the third ship to bear that name. The SS Weser was captured by HMCS Prince Robert off the Pacific coast of Mexico in 1940 and so could not have been interned in Spain. During the war the SS Weser was thought to have been responsible for the sinking of the British ship, SS Anglo-Saxon (sunk August 21 1940), but this sinking was later attributed to the German merchantman, SS Widder.
(at around 55 mins) Captain and several crew members are taking wave splashes/spray in their faces while standing on the bridge. The Captain keeps using his binoculars. If binocular lenses are wet they are completely useless for their intended purpose.
The airplane attacking the submarine at night, while it attempts to cross Gibraltar looks like a Hawker Hurricane. Gibraltar was home to 202 RAF Squadron which was equipped with Anti-submarine planes only, like Consolidated Catalina or Short Sunderland.
In the bordello sequence prior to the launch scene, swing music was being played. Under Hitler, swing or big band music was forbidden, and only German classical music could be played. However, popular music was common in such service clubs, especially outside Germany proper.
During the first encounter with a convoy, a lieutenant (the one who wears a sweater) gets out of bed with his hair a mess and in his face. After he climbs to the bridge in the next shot, his hair is perfectly combed and out of his face. This clearly indicates he owns and knows how to use a comb.
When the allied destroyer attacks, there are underwater shots showing the hull from below. The depiction of the fluid flow in these shots is incorrect. The waves around the hull move way too fast, revealing it's a ship model of smaller dimensions than a full-scale ship.
After returning to La Rochelle, and as the submarine base is attacked by allied airplanes, a presumably dead soldier can be seen lying on the ground that has his eyes wide open for a second, then notices that filming the shot has begun, and quickly closes his eyes to simulate death.