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.
Hmmm... Can't even think why this would occur, since the infernal soul requires that you deal damage before it explodes. I can only imagine that Syndra's W deals 1 damage (but cannot kill the unit) when she picks something up to ensure she's put in combat for tower aggro purposes?
I'lll have to get this bug into our database! Thank you.