Early in the game, Erica mentions a rumor that the enigmatic bartender, Boss, wears his Sunglasses at Night to avoid a Woman Scorned. After sleeping with Catherine and finding out she's a Yandere, it cuts to Vincent wearing a similar pair of shades while on a date with Katherine, who points out they do a lackluster job of hiding his face.
When Vincent sees that all evidence of Catherine has been removed from his phone, he says something along the lines of, "It's all gone!" and Orlando takes it to mean he's out of toilet paper. The Mood Whiplash in the scene really needs to be seen to be appreciated.
Earlier in the same scene, Vincent wonders if he's still dreaming. Orlando points out that Vincent would then be dreaming about two guys using the bathroom together.
In Catherine's Good/True ending:
"You promised you'd play video games with me tonight!" -Nergal, lord of the netherworld
"Fucking blocks. I'm gonna climb the shit out of you!"
During the credits in Katherine's true ending, the second picture has a series of cakes shaped like the blocks from the nightmares. But the funny part came from Vincent and Boss arguing with the latter giving the former a raspberry.◊
Some of the text reply options can be pretty snicker worthy. For example, when responding to Katherine in an early part of a game, you can say you're too tired and your brain's full of stuff. Which results in "NOT ENOUGH THINKING POWER. SHUTTING DOWN." Then shortly after that, Catherine asks if you want pictures of her. If you're going for a chaos route, one of the answers is "PIX NAO PLZ".
Catherine's true ending. Just Catherine's true ending. Especially when her father came.
That sounds wrong for some reason. Possibly because I've seen the true ending.
The random succubus in the post-credits scene who falls over out of sheer lust.
Many of the confessional questions are surprisingly funny. One example being, "What are your thoughts on underpants?" and the answers are, "It can go anywhere." or "You wear it... Under your pants."
A noteworthy one near the end is "Would you marry the perfect...robot?" One of the answers is "Is it...fully functional?"
A mixture of awesome and funny: the defeat of the Stage 3 boss (Immoral Beast). As the boss dissolves Vincent does his little victory dance and yells "Suck it!"
When Toby finds out that Erica used to be a man.
Toby: I want my V-Card back!!
Erica: Sorry but once that hole's been punched, there's no getting it back!
The freedom/neutral endings has Vincent arguing with Boss/Mutton over having Vincent's wish granted: Vincent wants Mutton to loan him some money...so Vincent can bet on a wrestling match.
He was actually asking for the money ($500) to bet on a wrestling match because a god somehow wasn't able to give him 50 grand initially to go on a space trip, which, in itself, is a pretty hilarious prospect. Then in the True Freedom Ending, you really need to see the dumfounded face on Mutton when he sees Vincent won the bet.
There's loads of Black Comedy in Catherine's bad ending; after becoming manic after getting dumped by Catherine, Vincent runs outside to find her. He gets hit by a car. He then runs about inside the bar to let everyone know he was hit by a car.
The fact that Boss/Mutton casually reveals that he was the one behind the nightmares and realizing that, that wasn't what Vincent was asking about is in itself hilarious, his face when he realizes this and him trying to backpedal.
His Boss Banter, which consists of hilarious lines like "Boy, stop moving!" and "Head goes BOOM!" Yes, they're cheesy, but they work.