Music preparation is never static.
Even a few weeks without rehearsal can profoundly affect the level of performance of the most polished pieces. The most challenging music requires daily interaction to be performance-ready.
Students can easily forget the dynamic nature of music preparation. It’s worth occasionally revisiting previously-performed music in rehearsal, to give students a concrete sense of this fact.
In other words, let students fail at music that they previously succeeded at: it keeps them humble, helps them keep the big picture in mind, and gets them back in the practice room to touch on the “old” music.