Using the Mutagenic Serum or Mutated Toe items on final boss Frank Horrigan will kill him instantly.
Both of these are well rooted in fact, but are still lies. The Serum can kill any normal Super Mutant, but Frank is quite frankly not a normal Super Mutant. Several NPCs will remark about the toe's ability, but this is a joke.
In the quest where Moira Brown asks you to irradate yourself, and gives you a special perk if you return with over 600 rads, you can get an even better perk. If you get the Ghoulskin Duster, which delays radiation effects by thirty seconds, and use it to report to Moira with over 1000 (fatal) rads, and you'll get the Rad Recovery perk, which gives you health for every point of radiation you take, and you won't die because Moira will heal your rads.
Using the Bethsoft 'GECK' tool to examine the game data files reveals that there is no Ghoulskin Duster, and there is no "Rad Recovery" perk only a perk that will give you limb healing if you get irradiated if you get 600 rads for Moira