Why does ahri's charm make my ult go on cooldown as Jarvan 4

Why? ...it happened 3 times...lol...same game... the horror.

Bug reports would be the best place to post this, especially with the most in-depth analysis of exactly what happened. The more detailed you are, the easier it is to figure out where a bug is.

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Most likely scenario if what you say is true though?

Every action a champion takes in LoL doesn't deliver it's buisness end on the same split-second the key-press is recognized.

Auto-attacks and abilities all play an animation when unleashing the attack or ability. Each attack and ability has a pre-set percentage of the animation it needs to play through before firing off a projectile or making melee contact. Usually about 0.5 seconds of animation. This way, arrows are fired after an arrow is notched in a bow and the string is pulled back and released, or when an axe meets a face, rather than the instant an animation starts.

After the arrow-loosed/axe-to-face part of the animation is reached, the rest of the animation is truely just-for-looks. You can cancel the second part with the Move command in most cases so you can do things besides standing in one spot.

Thing with cooldowns and mana costs is that those are often reliant on when you Pressed the button, where the actual buisness end of an ability comes ~0.5 seconds later. If you're unfortunate enough to eat a charm to the face in that 0.5 second window of starting the Cataclysm leap, you can get screwed over pretty bad. Same goes for a wide range of ultimates and abilities.