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I was playing Rammus, and taunted Lux as she was ulting (so we triggered at about the same time). However, instead of Lux cancelling her ult (which has a moment of charge-up), she continued to ult, rendering Rammus' taunt ineffective. To be clear, Lux still had time to jump up, spin her staff, and eject her laser all whilst taunted.
I will try to recreate the bug when I have more free time.