I noticed this today, as malz if you have your passive fully stacked so the next casts grants a voidling and you cast two spells at the same time (Q+W, E+Q, E+W, smartcasted and hitting the keys at the same time) It'll only count towards 1 spell cast as your passive. This wasn't how it used to be and i'm not sure when it was changed or if it was intentional.
Steps to repro:
1) Cast 3 spells as malz
2) Cast any combination of his spells at the same time, which, should leave you at one passive stack.
3) Watch as you end with no passive stacks.
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MoonsBane ?? Senior Member