When Maggie is painting the kitchen and spills the bin on Sweeney, in one cut the paint is covering all of that portion of his clothes while in the very next there is a spot that shows parts of his shirt. In the very next cut (when he says "lets hug it out") the paint is covering the entire portion again.
The movie gives the location of the Marine Base as "Fort Mahlus". Only the US Army begins their bases with "Fort" (i.e. Fort Benning, Fort Bragg, etc). The US Marines begin their bases with "Camp" (i.e. Camp Pendleton, Camp Lejeune, etc).
When Sam gives his wife his wedding band, he is wearing only dog tag. Soldiers are issued 2 dog tags, and when in combat 1 is worn on chain on around the neck and 1 is placed in their combat boot. This is done to easily identify their body, should it be needed.
When the DOD Police arrive at Cahill's house at the end, they arrive with lights and sirens blaring, alerting him to their presence and causing him to go outside and confront them. In real life, the police tend to respond to domestic disturbances silently without lights and sirens on in order to prevent this sort of thing from happening.
When Tommy goes to Sam's house after his release, there is some kind of tape stuck on the back of his jacket while he's standing at the door. After walking inside in the next shot it has been removed.
Marine bases tend to be on the coasts, since the Marines are a naval service. But the base Sam is at is portrayed as being far inland. The two main Marine bases in North America are Camp Pendleton and Camp Leujene, which are both located in warm areas, whereas the film is set in a snowy, cold locale.
In the beginning, dated in the movie as 10/7/2007, Sam says he's leaving "in 4 days" (10/11/07). Later, at the dinner table, he tells his brother he's leaving "Tuesday". 10/11/2007 was a Thursday.
In the beginning of the movie, little Maggie, tells her grandfather, Hank, that her birthday is "in the spring." Hank agrees with this, commenting that her birthday is March 10. However, March 10 is still winter. Winter ends March 21.