Continuity mistake: While Maria sings "I feel pretty", she leaves the pink cloth next to the edge of a table. When the angle changes its position differs.
Continuity mistake: When Bernardo catches the basketball, the Jets are divided on two separate groups. Half a second later the next angle shows a closed single group.
Continuity mistake: During the rumble, Riff slashes at Bernardo's shirt with his knife, and he spins away but you can't see any tear in his shirt. Then, in the next shot, when Riff is running toward him again, there's a noticeable cut in his shirt (on the back).
Continuity mistake: At the gym during the dance, Baby John tells Riff, "Pick up" when Bernardo and the Sharks come into the gym, and all the Jets stop dancing - except for Ice and Velma. Then in the next shot, Ice is suddenly in the big Jet mob, and Velma is off to the side with Graziella.
Deliberate mistake: The knives at the rumble bend EVERY time they move.
Continuity mistake: In the playground, when the Jets stand up, the guy with the violet t-shirt stands up twice.
Continuity mistake: In the begining, when the Jets are snapping their fingers, the guy with the blue t-shirt has his arm against the fence. In the next shot he doesn't.
Other mistake: In the prologue, after Riff stands up and signals to the other Jets to follow him, they all stand up, then in the next shot you can see Baby John stand up again.
Audio problem: During the "Officer Krupke" song, when Riff is sent to the analyst, just before the start of the verse, someone says "whoops" and it isn't part of the song.
Continuity mistake: At the bar, Riff's tie when challenging Bernardo swaps from tight to loose between shots.
Continuity mistake: A spider web drawn by a girl near the fence disappears.
Continuity mistake: In the beginning, an overview shot of the playground and the Jets is shown, the movie then cuts to a closeup of Tony snapping his fingers. In the closeup, there is a boy riding his bike past Tony that was no where to be seen in the previous shot from above.
Continuity mistake: In the basketball field, the guy with the white t-shirt and pink vest is laying against the fence. In the next shot he is slightly turned to the right.
Continuity mistake: When the camera is zooming towards the playground, there's a guy on the right climbing the fence. He disappears from the fence half a second later.
Continuity mistake: When Glad Hand is telling them about the get-together dance at the gym, you can see A-Rab putting his arm on Action's shoulder, then in the next shot he does it again.
Audio problem: After Anita discovers Tony and Maria's tryst, Maria says "Now you know." She completely botches her Puerto Rican accent. She practically sounds British.
Continuity mistake: During the number, "Gee, Officer Krupke," Ice is standing right behind Riff, right before he sings "Dear kindly judge your honor..." then suddenly he's in the doorway of Doc's candy store.
Continuity mistake: When Maria finishes singing "I feel pretty" the purple fabric rests around her left shoulder. A frame later, from a new angle, it's resting around her arm.
Continuity mistake: At the bar, 2 of the darts are very close together in one shot, and farther apart in the next.
Revealing mistake: When Maria is cradling the dead Tony you can see Tony's chest heave as the actor takes a breath.
Answer: People would not come to the theater to see Marnie Nixon despite the fact that she has sung the lead roles in West Side Story, The King and I, and My Fair Lady. They'd rather see that darling girl from "Miracle on 34th Street" again (or Deborah Kerr or Audrey Hepburn) than an unknown with actual talent.
Myridon