__(**Hotfixed**, now overheal works properly)__
Im not sure how's the interaction. But champions that have a passive that activates after death triggers that bug, giving them a lot of shield.
The shield itself vary related to the champion in specific.
Those champions are affected by this bug:
* Anivia (with his pasive, the egg can have up to 3,5k of shield)
* Sion (Shield strenght may vary, but once he attacks once, the shield resets to a normal shield that the player could gather normally)
If any of those champions dies with his passive up, the shield will appear. This, of course, can only happen each 3 min on Anivia (which is her passive CD) and with Sion always will happen, but its very unconsistent on shield given.
Here's a gif with overheal shielding anivia on her passive:
Here's a video, with higher resolution and smoother fps (you'll have to click it):
Those are the runes i used to get that bug:
(Credit to "Roo Pbe Fus" for finding this out)