Unprepared high entrances for Brass
High entrances out of the blue are a nightmare for every brass player and particularly tricky for horns (due to the way they produce their tones). The chances are quite high they the tone will be missed or there will be a split in the unison sound or the attack of the tone is not spot on. This is practically gambling for the player and is nothing that can be overcome with professionalism. Even the best players on earth will miss some of these entrances. Without going too much into detail why this is happening, the essential thing is that you try to avoid that unless it is absolutely necessary. It makes life for your players way easier if you gradually lead them to these high notes. Have a mid high entrance one or two bars earlier and gradually move them higher up will make the chance of them hitting it right much higher.