In the final boss of Sunqua Peak (not CM).
When: During the lightning phase when you need to collect orbs to jump and dispel the storm clouds.
What: The phase doesn't finish even after all the storm clouds are dispelled.
Repro: Not sure. We managed to get our instance into a state where the fight would always stall on the lightning phase. The boss's name was in yellow like a neutral mob, but that might always happen.