Posted this in Bug Reports forum, felt it needed to be posted here as well just to ensure it gets the attention it NEEDS immediately.
When having a Tear of the Goddess, the 3 second cooldown period between spell casts is NON-EXISTENT at the moment. It is possible to spam and gain all the mana from each spell cast.
Reporduction steps: Purchase a Tear of the Goddess, Manamune, or Archangel's Staff
Proceed to use two spells near simultaneously, paying special care to the mana you gain from the passive.
If you cast two spells, you will have gained 8 mana. Three spells, 12, so on and so forth.
This can potentially completely break champions like Ez and Karth and Urgot for the moment, and honestly it cost me a defeat in a ranked game where an enemy MF spammed abilities and had a Manamune transform by roughly the 12 min mark. It's not fun, trust me.
You get a hidden second "stored cooldown". So for example, you can Lay Waste twice and get 8 mana easily. The 3 second cooldown starts after the first ability cast, so unless your cooldown is literally 1.5 or less, you'll be able to get 3 charges from casting 3 abilities in a row.
I just played a game as Singed and didn't just get 4 mana per second all game, so I don't think this bug actually exists.