Revealing mistake: In the dance scene, when the two gangs are "battling," if you look in the background at the Sharks side, you can see Maria among others mouthing the rhythms of the songs. (00:37:25)
Continuity mistake: In the bar, when the cop is asking where the fight is going to take place, he addresses a guy in the back. His position and that of his tie changes in half a second when the angle changes.
Deliberate mistake: For dramatic and cinematic purposes, the floor of the court where the last scene takes place is wet, as opposed to the circle where Tony dies.
Continuity mistake: When the gangs are fighting in the playground, in the aerial shot you see that two of the seesaws are pointing up away from the gangs. When the Jets leave a few minutes later, all of the seesaws are pointing up away from where they were originally. (00:14:16 - 00:17:39)
Continuity mistake: In the prologue, after the gangs jump over each other in the alley, Snowboy and Baby John both chase Bernardo, while A-Rab and Action go the other way to chase a different Shark. In the next shot, they are all together chasing the Shark A-Rab and Action were following.
Other mistake: During the dance at the gym, Maria doesn't snap each time she should have. Natalie Wood was unable to snap, so it was dubbed (as well as her singing, which was dubbed by Marni Nixon).
Continuity mistake: At the dance, when everyone is swaying with their partner, you can see both of Ice's hands on Velma's back, then in the next shot when it's following baby John, you can see Ice put his hands on her back again. (00:32:30)
Continuity mistake: When the camera zooms into the basketball field, the shadow of the Jets points in a 8 o'clock direction. In the close ups it's a 2 o'clock.
Continuity mistake: After the Jets and the Sharks have challenged themselves, Tony takes his right hand to his tie, but the immediate shot shows the hand by his side.
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