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.
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 Tommy is reading the local newspaper reporting the retrieval of PFC Willis' body from Afghanistan, the first line of the article refers to the "United States Marine Corp". The correct spelling is Corps.
The goof items below may give away important plot points.
When Sam visits the grave of Joseph Willis, it states Willis' rank as "PVT". In the US Marines, you are only a "Private" (E-1) for the first 6 months of enlistment or if you are demoted to the rank. After 6 months you are now a "Private First Class" or "PFC" (E-2).
When Sam holds the gun to his head, about to commit suicide, the hammer on the gun isn't cocked. Though, the Beretta 92FS being both double-action and single-action, it is still unlikely for him to first chamber a round, then de-cock the gun.