When Dito (Shia LaBeouf) and Mike (Martin Compston) are talking about what music they should play, Mike says that they should sound like Black Flag or Major Conflict. Major Conflict is a NY hardcore band in which the director of the movie actually played when he was young.
In the scene where Monty (Chazz Palminteri) has a stroke, Antonio (Channing Tatum) throws a table through the glass window of a door. This was improvised by Channing Tatum, who got so wrapped up in the scene he nearly lost control. The rest of the cast stayed in character and finished the scene, and the director liked it so much he kept it as the final version.
In one shot of the subway, American flags are visible next to the subway windows. These were added after the attacks of 9/11, years after the movie takes place.