[This is a repost. Considering there's not even a notice thingy on the client regarding it, I can only assume it was not yet brought to Riot's attention. Deleted previous thread]
Casting E while it's mid-air to make it explode as soon as it stops will consume mana an additional time, for a total of double the mana cost.
100% reproduction rate. Should fix ASAP, as it's pretty impactful.