Other mistake: After Jody, Charlie, and Cooper take their last curtain call, the shot goes backstage. Sergei yells at Eva, "You! I'm gonna kill you!" presumably because she surprised him by taking Maureen's place. Eva responds, "I'm so sorry. I wanted to tell you." And they argue playfully and then hug. But it's been a long time [the entire length of Cooper's ballet] since their ballet finished. And we saw the backstage scenes immediately after Cooper's ballet and after the Italian Symphony ballet, and everyone had a chance to talk right then. And Eva and Sergei finished their ballet in each other's arms. So they should have had that whole conversation a long time earlier. Even if real ballet dancers wouldn't be permitted to talk after a performance, the dancers in this movie are shown speaking freely to one another backstage as soon as the curtain falls.