Corrected entry: When Chino shoots Tony as he is running into Maria's arms, there is no evident bullet wound anywhere on Tony. If Chino shot him from the side, it would have been visible. If Chino had shot him from the back, it would have been visible. If Chino had shot him from the front, Maria would've gotten shot too. It is also worth noting that Maria doesn't seem to notice or care that Tony's been shot, and instead of becoming shocked or dismayed she treats the situation with a fairly nonchalant attitude.
Corrected entry: At the dance when the gangs begin the circle walk, the girl in the blue dress walks the wrong way. She bumps into another girl, then quickly turns around and walks the other way.
Corrected entry: When the Jets are singing their first bit in the tonight quintet ('The Jets are gonna have their day tonight') If you listen carefully you can hear that one of them gets the words wrong and sings 'way' instead of 'day'. It's on the soundtrack too.