Urgot and Tear of the Goddess
I'm an Avid Urgot player and have noticed improper functioning of Tear of the Goddess in such a way:
The item does not add stacks upon spell cast or mana expenditure, at least for the first cast; it will usually take two casts to get the stacking effect rolling.
Also, it seems as if some sort of timer within the item resets if you don't actively keep the stacks rolling (i.e. expending mana or casting spells). This makes a an already difficult, unforgiving champion even more so and less efficient.
ALSO, I am fully aware of the "can only occur every 8 seconds," and actively watch for this CD on the item so as not to waste mana.