What? An odd "working as intended"...
Simply put, Darius activated his ult on Vayne and was in mid air, waiting to come down. At this point our Vayne used her knock back while our Leona simultaneously used her stun. Darius was pushed up against the wall, so effectively double stunned. Well, his ult still went through and killed Vayne while he was pushed up against a wall with a stun icon above his head, and without ever actually finishing the ult animation. This perhaps needs some looking into as it makes no sense at all. Anyway, just a heads up!