When Mélanie leaves the studio before the performance for the American, she is wearing her evening dress (halterneck and bare at the shoulders). The next shot shows her walking through a passageway with the plain dress she wore at the concert some days earlier. The next time we see her, she has a coat on over the original halterneck evening dress.
In the last dinner the bottle of wine changes from the left to the right between shots.
Melanie encourages Tristan to learn a specific Bach Prelude as a surprise for his father's return; at the actual concert, he plays something else (a composition for this movie, I believe).
During the radio concert at one point a hand can be seen reflected in the piano beating time, despite the fact that the group is shown to be performing without a conductor. In the subsequent shot, showing the opposite side of the piano, the dolly track, and the foot of a camera operator sitting on the dolly can be seen reflected in the piano.
When ten-year-old Mélanie enters the exam's room with the jury, the reflection of the boom mic is clearly seen on the black piano.