When Gig and Buck first approach Gig's father's grave, there is a mound of fresh dirt over the site, obscuring the bottom part of the headstone. In a subsequent shot, the headstone is much more exposed, with its base visible. (It is also odd there are no dates on the stone.)
When the boys are running away towards the end and meet in the cemetery, there's a part where a man can be seen walking across in the background.
Mr. Murphy states he was in England in 1917 with the 77th Infantry Division. That division did not move to Europe until April 1918.