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 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.
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.
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.
On September 11, John finds a TV covering the attacks. In the first shot, the North Tower is clearly the only one still standing, meaning the South Tower had already fallen. But after a quick reaction shot to John's face, the TV then shows the South Tower falling.
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.
When John takes Savannah home to meet his father, and she puts on the glove to look at the coins, she puts it on her left hand. Then, later on when she speaks to John, she has the glove on her right hand.
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.
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.
After his father's funeral, when John is at Savannah's house and he's listening to her talking on the phone, his rank is upside down on his shoulders. A soldier would never put them on his uniform incorrectly. Later they are on his uniform the correct way.