When Jim is getting dressed in the hayloft for his date, Slim jokingly refers to him as "Mr. Hemingway". When the film was being made in 1924-25 Ernest Hemingway was becoming famous, but in the movie's time frame of 1917, he was still unknown.
In the recruitment parade scene, several women are wearing drop-waisted dresses with hems that end well above the ankle. This is appropriate for the year of production, 1925, but quite anachronistic for the time in which the scene is ostensibly set, 1917.
Early in the movie, we see James Apperson announce that he has enlisted. His father, who had been sternly lecturing him moments before, comes up and congratulates him. Suddenly, the father now has a lit cigar in his mouth, with a long ash, indicating he's been smoking it for at least a while. But all the time prior, we saw no sign that the father had a lit cigar anywhere on him or near him.
The three future recruits live in New York City (one works construction on a skyscraper, another works a bar in the Bowery), but during the recruitment parade, there are shots of wide avenues lined with low buildings and palm trees - clearly shots of 1920s Los Angeles.
The soldiers march past eucalyptus trees during the Belleau Wood sequence. Native to Australia, eucalyptus trees are not found in that region of France. They can, however, be found in Griffith Park, Los Angeles, where the sequence was shot.