If you have negative max hp (e.g. from getting a bunch of siphon stacks on you) then you're immortal.

[0.5772156649]

I don't think this is practical to do in game, but if an enemy player permanently steals enough max hp from you with siphon bullets that you have negative max hp, then you become immortal and can't die from damage.

 

[MyNameIs8Ball]

You can do this to the Abrams punching bag in the hideout. That sounds neat but this bug also prevents on-hit effects from working, so you couldn't test Mina's Love Bites or Graves' Essence Theft, for example.