During the climatic dinner scene with Savanna after returning for his father's funeral, John is wearing his staff sergeant rank on his shoulder epaulets upside down. (The rocker should be furthest from his neck). Halfway through the scene, it shifts to the correct positioning, and then when he leaves, it is incorrect again. This is the only time the rank on his uniform is applied incorrectly.
Where John fire his weapon at a terrorist, the view of his weapon changes from a M4 to a M249 SAW and back to an M4.
Tim is taking the trash out and John drives up to speak with Savannah who is not home. Tim reaches into the trash and gives John a scrap of paper to write a message on to leave for her, and a pen also magically appears.
When John visits his father at the hospital, a man with a maroon t-shirt walks into the aisle where his father is lying. He does this twice, first turning left and then turning right.
At Savannah's party during John's leave, Randy offers John a bourbon. John takes the glass from him twice.
The postman places John's letters to Savannah in the P.O.Box from the front. The front of these boxes has a combination lock that is meant for the box-holder to use. Mail is placed in these boxes from the open backs of the boxes.
When John and Savanna are sitting in the kitchen talking, his left shoulder rank insignia is upside down.
SSG Tyree is shown wearing the blue infantry cord and discs only soldiers that hold an 11 series PMOS (Primary Military occupational Skill) can were them. If SSG Tyree is Special Forces he would hold a PMOS in the 18 series. AR 670-1.
When John is in Hungary when he hears about 09/11 (as indicated by the signs on/near the coffee shop: Tigris Kavehaz, Muvesz...), and the TV news are in Serbian (even using Cyrillic - not Latin - script). This scene is not necessarily a mistake. Although not well explained in the movie, in the book John was stationed in Kosovo. He could simply have been standing outside a Hungarian-style coffee shop there, which explains the Hungarian language on the store front. Serbian is recognized as an official language of Kosovo, which explains the TV news being in Serbian.
In the scene after John's dad passed away while at Savannah's house at dinner, John's rank is upside down throughout the scene.
There is a typo in the end credits. In the Special Thanks section it reads "United States Mint And Its' Philadelphia Facility"
The watch John wore in the movie (Suunto X9i) is very popular in the military for it's GPS capabilities. The first time John can be seen wearing this watch is in scenes from 2001, however this product was not produced until late 2006.
In the beginning of the movie, it is spring 2001. This shows up after the opening credits, at the party at Savannah's house. Then, it is Christmas (when the soldiers are distributing the mail, they say 'Merry Christmas'). Then, 9/11 happens. This can't be right, because Christmas 2001 has passed.
In one scene of John in his room when he's at war, it is around September 11, 2001. They have a poster on the wall of a Bud Light beer bottle, and it says Drinkability on it. But the term "drinkability" wasn't actually used by Bud Light until recently.
Where John fires his weapon at a terrorist, it is not the sound of a M-4 but more of a .50cal Machine gun.