The item "skimmed milk" sold by Kurd Butterhoof in High Mountain stacks when used in different bag slots.
For all practical purposes you are invincible in-world. Fun for world pvp and world group content (no one will die with you healing at 1000%+). Invaluable for leveling.
(For those who know this about this I held off for as long as possible but there are now multiple bug reports on the official forum, some of them are even coherent).