This bug is old, very old. But ill post it there since it stills happens.
Basically if you manage to hit an autoattack when you are resuscitating, you will be able to heal from life steal.
1. Pick any champion (ranged champions are better for bug reproduction)
2. Get some life steal items + Guardian Angel
3. Get killed with GA on while you are attacking an enemy champ
4. If you die and the autoattack hits after you die, youll get healed related of how much life steal you have
Here's the video of the bug:
And this video is by comparision of how GA works:
Credit to Caenen to let me know it still happens.