Juniper's Knot: When the tired boy falls asleep in front of the demon girl, she is suddenly struck by her bitterness and frustration on how she cannot sleep, escape, or in any way find respite. The emotional baggage makes her lose her composure and throw every profanity she can think of at the sleeping boy, even though he can't hear her.
In Dual Destinies, Apollo, who hasn't cussed once in the entire series until this point, suddenly breaks under the fact that his best friend has just been murdered.
Apollo:DAMN IT! IT'S NOT FAIR!
Then he does it again, twice in the same statement, when he learns that Detective Fulbright was the one who killed his friend.
Apollo: That scumbag even made me turn on Athena...! Damn it! How the hell did I fall for all that?!
Nine Hours, Nine Persons, Nine Doors has a a good number of uses of strong language, including "fuck", although most of them are said by the same character, Santa. However, when they're in the incinerator, Lotus, who's completely in the dark about the backstory to what's going on, drops one of her only strong swears.
Lotus: "What the hell?! What! The! Hell?! What are you talking about?! It's "9 years" this, and "9 years" that, and when it's not 9 years something, then you're talking about some sort of fucking "experiments"! What in God's name are you talking about?!
When the surviving characters are running up the stairs to escape in the true ending, Junpei hurriedly explains to Seven and Lotus about hexadecimal, bases, and about what would happen if you go all the way to base-22. This leads to the two finally realizing the big twist revelation