Hi
Screenshots below
This was posted on the LoL bug forum, i just tested in a custom game and it is still working on OCE server. When playing Amumu if you take 0 or less damage from a hit you can stack the 50hp Runic Shield infinitely.
How to -
1. Pick Amumu.
2. Have 1 point in the Runic Blessing mastery.
3. Max ability E.
4. Build armor in game.
5. Once E is maxed and you have armor you need to die to reset Runic Blessing (this is a very importance step, this bug with keep stacking the shield only if you already have it active).
6. Find a bunch of minions and stand there and let them hit you, for each hit you take you'll get another 50hp to your shield.
It worked for me when i had E maxed and had built a Frozen Heart (not sure how much armor is the minimum, thats just when i tested). I'm guessing mine got to around 30k hp in a matter of minutes.
Just a small point, the screenshots below were done on a second test where the group of creeps was much smaller so the stacking was slower, the more creeps hitting you the faster the shield will grow.
Enjoy,
Selma