When Nick runs into his buddy from pre-prison, the friend mentions rerunning into someone from his old unit. Then later mentions he's not in the Army anymore, but his friend is wearing a Marine uniform.
In the first confrontation with the Seizu, Nick is pistol whipped a single time on the left side of his face. In the next scene, his left eye is bruised, but he also has an unexplained cut above his right eye, which could not possibly have been caused by the same blow.
When Nick steps outside of the bar after being with his brothers, you can hear a helicopter and in that shot you can distinguish the silhouette of a bell UH-1 Iroquois, which was not taken in use before 1959, 5 years after the movie takes place according to the titles earlier on in the movie.
Jared Leto congratulates Pauline on his 'stripes'. Pauline is a commissioned officer, only non-coms, sergeants, have stripes.
The car driving Nick around throughout the movie is a 1960 Chrysler LeBaron.
This movie's setting takes place 9 years after the war, in 1956.
The opening shot features large letters across the screen saying, "Osaka, 1954" while simultaneously showing a 1960 Chrysler Crown Imperial Le Baron.
Early in the movie, when the waitress hands Nick Lowell (Jared Leto) a beer, it is a long neck Bud instead of the stubby beer bottles that were used when the film is set.