When Harry and Jo are dancing together in the coffee shop after seeing Jimmy off on the train, a curl of Jo's hair comes loose and falls onto her forehead. At one point in the dance, when she turns her back to the camera, you can see her try to brush it back into place, but it's still loose. Immediately after the dance, though, as the cashier is applauding, the curl is firmly affixed back in place.
At the start of the film, the date of 1916 is shown. Later in the film it mentions the liner Lusitania being sunk. The timing is off because the ship was sunk in May of 1915.
Depicts a Southern Pacific Railway "cab-forward" steam locomotive hauling a passenger train. That model locomotive was used in freight service only.
Jo's brother was listed as "killed in action" on September 28, 1917. The first American soldiers to die in combat died on November 3, 1917.
When Eve is in her apartment, she ushers Jo out the back and dashes to answer the door, she puts her lit cigarette in an ash tray and it immediately rolls to the floor in full view.
Jo's YWCA uniform (like much of her clothing in the latter half of the film) is characteristic of World War II, when the film was released, and not World War I, in which it is set. Her uniform's shoulders are too broad and the skirt too short, and she is wearing sheer stockings and pumps instead of opaque stockings and high boots or oxfords.
A Southern Pacific "cab-forward" locomotive is seen pulling the train at the start of the movie. It is shown stopping in Iowa to let the main characters disembark. The SP does not, and never has, run into Iowa.
Newark is in New Jersey, not New York. There's no way any sane person could have interpreted Eddie's telegram to mean the Palace in Newark.