Visible crew/equipment: When Sean Penn is outside the motel with the gun, he fires three shots past Benicio Del Toro's head into the ground behind. When Benicio falls to the ground you can see an ear plug in his left ear. (01:49:05)
Continuity mistake: When Paul is introduced at the hospital, in one shot he thinks, "What am I doing in this precorpse club?" and turns his head to his left. In the next shot, he is turned to the right, and the shot after shows him turning from his left to his right. (00:04:45)
Continuity mistake: At the beginning of the film, when Jack is playing Jenga with the teenager, Jack crosses his arms twice. (00:01:45)
Continuity mistake: Cristina switches from looking down to looking at Paul after he throws his cigarette out the window in the car outside her house. (01:33:35)
Continuity mistake: As Cristina hangs up on Ana at the bar, Cristina's left arm shifts from across her lap to across her chest. (00:41:00)
Factual error: In the movie, Cristina declines to press charges so Jack goes free, but it's the District Attorney who decides whether charges are pressed on behalf of the state.
Continuity mistake: Inside the church, when everyone's singing "I've got a river of happiness" (or something like that), Benicio Del Toro exchanges glances with a young man. Watch the black man to the right of Del Toro, his mouth is in no way synchronized to what the people are singing. (01:23:15)
Continuity mistake: Right before Sean kills The Man he pulls out a silver gun and puts it in his pocket. Later on in the movie he has a black gun.
Answer: The title refers to an experiment in 1907 which attempted to show scientific proof of the existence of the soul by recording a loss of body weight (said to represent the departure of the soul) immediately following death. Referred to as the 21 grams experiment as one subject lost "three-fourths of an ounce" (21.3 grams), the experiment is regarded by the scientific community as flawed and unreliable, though it has been credited with popularizing the concept that the soul weighs 21 grams. (Wikipedia).