Whenever Anivia gets into her passive she gets a ridiculous janna-like shield that is MASSIVE. We three man ganked her, got her in passive, didn't even get to damage her in the passive because of her shield, she got a kill, then just walked out still having a ridiculous amount of shield left. I tested this in a custom once with overheal and once without and this only works with overheal. Also, whenever she procs overheal normally she also gets a huge shield though the shield is not as big.
As you can see in that screenshot, there is no shielding champion on her team and the overheal animation is on her and her shield is huge. She got a shield like the one in the first link every single time she went into passive.
EDIT: I tried again today and it seems like its fixed, the anivia passive acted normally and overheal didnt give oversized shields.