The lookout proclaims that the torpedo is closing on the starboard bow (the Lusitania was hit on the starboard side). However, when Professor Holbourn and others go to see it coming, the smokestack is tilted as though they are looking for the torpedo on the port side.
During the first scene of survivors in the water, a particular extra is noticeable. The same girl is in a following shot a few moments later during the chaos on deck, still dry.
On at least two occasions the captain of the U 20 gives the order to "blow ballast" when he wants to dive the submarine. This order would result in all water being blown out of the ballast tanks and should only be given to surface the boat. In order to dive a submarine one must flood the ballast tanks with water.
Early in the movie at the docks in New York the American flags have the staggered arrangement of 50 stars. During WWI the American flag only had 48 stars in a rectangular arrangement.
The U-20 is shown to have four forward torpedo tube doors. However, it should only have had two as the Type U-19 only had two torpedo tubes; the interior correctly contains two torpedo tubes.
The Lusitania's dining salon was a two-story setting with a dome, and its walls and ceiling were painted white.
When Prof. Holbourn and Avis first meet, he asks if the deck chair next to her is taken. Later, they are shown seated together in the dining salon. On the Lusitania, all seating arrangements in first class were pre-assigned (a process that continues on cruise ships in the 21st century), as were most deck chairs. It's unlikely that Holbourn and Avis would be suddenly supping together, or that Holbourn would be gauche enough to take another passenger's assigned deck chair, even if it was empty.
In the early part of the movie, John Hannah is talking about the departure of Lusitania from New York, he distinctly says something about heading 'west' toward their destination. Sailing from New York to England/Europe, one has to be sailing EAST, not west.