After his son dies, Louis, (as he's near to losing his sanity from grief) can't help but think of dark jokes.
When he buys the coffin, in his mind "an announcer suddenly spoke up cheerfully: I got my kid's coffin free, for Raleigh coupons!"
When his friend says that they should meet for lunch, the idea seems so alien to him that it reminds him of Sci-Fi novels: "The natives here on Planet Quark have an odd custom when one of their children dies, Lieutenant Abelson: they meet for lunch. I know how grotesque and barbaric that sounds, but remember, this planet has not been terraformed yet".
And a bit more Black Comedy: after Louis has finished digging up Gage and is carrying his body, wrapped in a tarpaulin, back to his car, a dog starts howling loud enough to wake its owner and their neighbor across the street. Louis panics and hides behind a tree while this exchange takes place:
Owner: "Shut up, Fred!"
Neighbor: "Shut him up, Scanlon, or I'm calling the police!"
Louis: *thinking* Goddamn you, Fred...
Surrendra Hardu, the doctor that has the night shift at the university, describes an incident where an incredibly drunk girl with a large cut on her thigh is brought into the infirmary, and while he is stitching her up, she pukes on his head.
While Rachel and the kids are visiting her parents for Thanksgiving, when she calls Louis he hears that Gage apparently wound up escaping while he was having his diaper changed and wound up pooping all over Rachel's father's study. Given his relationship with his father-in-law, Louis can't help but smile and think "thatta boy, Gage!"