Before Oscar is shot on the station platform, there are dozens of witnesses surrounding the scene and tussling with BART officers. After he is shot, nearly all bystanders have disappeared.
After Oscar has told Sophina about losing his job and they kiss, he moves his hand to her neck in close-up, and again in the next medium shot.
At 4 minutes 30 in, Oscar puts a bulky bag of weed onto a high shelf in the closet. At 18 minutes in, Oscar retrieves the same bag of weed, which is now flatter and less bulky, from the closet shelf and slips it down the front of is pants.
At the start of the prison scene, the text on-screen reads "San Quentin Federal Penitentiary". San Quentin is a state prison.
Clearly in December any year trees should not bear leaves Bay Area.
When Oscar is in the kitchen at home, he looks at a calendar on the refrigerator with a rent date marked. It is supposed to be December of 2008, just before he died on New Year's day of 2009. The calendar is showing November of 2012. . . the year the film was made.
When Oscar's mother visits him in prison, the text on-screen reads "New Year's 2007." Oscar mentions seeing WALL·E (2008) with his daughter. That film was released in June 2008.
The BART system maps inside the train cars were introduced in September 2009. All of the in-train scenes take place on December 31, 2008, and January 1, 2009.
Right after Oscar finishes talking with his mom, on the morning of December 31, 2008, he drives by a crowd. One person in the crowd wears a Barcelona jersey from the 2010/2011 season.
When Oscar drops his daughter off at daycare, he has to knock on the door to enter with her. When he picks her up later, he walks up the side driveway, enters the back yard and starts playing with her. No one really acknowledges him.