Averted: Bob is able to survive what most humans can, and unable to survive what most humans can't.
"We need to show how strong Bob is. Let's make him survive six shots to the chest!
"I want to make a humorous scene in which Bob is inexplicably able to survive what no human can."
Lampshaded: "What are you, made of iron?" "Nope, just my blood."
Invoked: Alice creates a vitamin that causes those who consume it to gain the ability to survive the most impossible attacks.
Exploited: Bob uses his exceptional resilence to become a Stone Wall.
Defied: Charlie exposes Bob to radiation that causes Bob to lose his invulnerability.
Discussed: "This guy is unstoppable!"
Conversed: "Oh, is this the one where Bob is invulnerable for no reason?"
Bob believes he is superior to the rest of the human race because of this unexplained invulnerability and proceeds to take over the world. Military from all around the world are unsuccessful in trying to stop him, and he eventually reaches his goal.
Bob is literally made of iron, and the oxygen in the air causes him to rust, slowly rendering him useless.
Bob does not heal properly due to the sheer volume of damage he receives. He eventually dies from all his wounds.
Bob's durability is given to him from stacked up pain or repeated injuries from past times.
Bob isn't durable at all, he just has an incredible tolerance for pain.
The reason Bob is so durable is because he isn't human.
Bob uses his invulnerability to stop evil in the world.
Bob trains his body to the point where he can take more damage than most humans.
Played For Laughs: Bob needs to regularly apply lubricant to his skin to prevent it from rusting.
Played For Drama: Bob's invulnerability results in him coming across as rather cold because he cannot sympathize with the injuries of others.