They did something to her earphone monitor and she missed one of the notes. Diana was always pitch perfect so that was the only way to mess with her.
You KNOW the “Idol Song” will be written for Adam and have really high notes and screams built in so Kris will have to struggle through it.
I don't know if Diana is just being classy about it, but she's said that there was a known "dead spot" on the stage in terms of the monitors, and that she accidentally wandered into it, forgetting it was there.