If your mundo and you ult right before you die, with guardian angel up you will come back with more life then intended, as it apears mundos ultimate does not "stop" when it "dies" so you will come back with more then the 750 cap. read about it in the forums aswell that people are complaining abut just this one, and sience riot doesnt seem to react on it i guess thats how the mechanics works for mundo
Read the post here Mundo's Ulti + Guardian Angel bug - League of Legends Community
If you happen to confirm this bug please type it here and perhaps would be fun to see a video about it