Infernal dragon soul does a burst of damage on autoattack or spell proc. If you W a minion with Syndra and it has low enough HP, it will instakill the minion and put Syndra's W on cooldown, preventing her from throwing it for the damage/slow. It seems like a unlikely thing to happen, but it messed up 3 of my W's on my very first game of having infernal dragon soul and was honestly kind of frustrating.
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