Sicker Puns

He eventually died. I guess the condition was terminal.

The man never took it seriously at first, he figured he was just getting older and blamed it on age.

After a few weeks, the man has developed an incredible frequent and annoying cough.
His wife is annoyed and is constantly telling him to go the doctor, but the man kept refusing.

One day during an argument, his wife has had it with his coughing and hacking and tells him "Im making a bet, if this damn coughin kills you i'm writing ' I told you so' on your tombstone!"
The man laughs her off since they both have a twisted sense of humor, and tells her its a deal, if the coughin kills him she can carve that.
The man continues on for another week

One day the man is out going for a walk through his neighborhood, when a freak accident occurs between a truck carrying coffins and a car, which results in a coffin flying off the truck, tragically landing on the old man and kills him.

Later at his funeral, his wife makes a very odd request to have them carve "I told you so" on his headstone.

When the caretaker asks her why she wants to do this, she tells him about their dark humor, and fills him in on the bet they recently made.
The caretaker is touched by the story, and agrees to do it for her, because in the end,

It was that damn coffin that killed him