Factual error: In the many scenes where Marion, Harry and Tyrone are injecting drugs, the pupils dilating is a mistake. The drug used in the movie is heroin, which constricts the pupils.
Continuity mistake: Harry stands on top of a building with a girl folding paper planes. The plane they fold is folded with a blunt front. He throws it off the building and then the plane flies, all of a sudden the nose is sharp, this is visible.
Director Darren Aronofsky included nearly the entire cast of his debut film Pi in Requiem for a Dream. Abe Rabinowitz, Arnold the shrink, Nurse Mall, the mailman and the E.R. Doctor were all in Pi. Joanne Gordon even plays the same character Mrs. Ovadia (in Pi she was the evil landlady and is one of Sarah's friends in Requiem). Also his mother Charlotte Aronofsky is Mrs. Miles, another of Sarah's friends; and his father Abraham Aronofsky is the newspaper reading man on the subway.
Sara Goldfarb: I'm somebody now, Harry. Everybody likes me. Soon, millions of people will see me and they'll all like me. I'll tell them about you, and your father, how good he was to us. Remember? It's a reason to get up in the morning. It's a reason to lose weight, to fit in the red dress. It's a reason to smile. It makes tomorrow all right. What have I got Harry, hm? Why should I even make the bed, or wash the dishes? I do them, but why should I? I'm alone. Your father's gone, you're gone. I got no one to care for. What have I got, Harry? I'm lonely. I'm old.
Harry Goldfarb: You got friends, Ma.
Sara Goldfarb: Ah, it's not the same. They don't need me. I like the way I feel. I like thinking about the red dress and the television and you and your father. Now when I get the sun, I smile.