The author pushed to allow B.D. to swear ("SON OF A BITCH!") when he wakes up after losing a leg in Iraq. This is significant because newspaper comics have extremely strict clamps on their subject matter. A few papers dropped the strip, most simply edited the line; a few kept it as-is, with the paper's editor stating it was justified.
Melissa saying "That's sergeant bitch to you", which seemed to just be for the sake of the punchline.
The only time there was actually a curse spoken in the strip was also 1986, where upon seeing that "Be" only got him two points in Scrabble, Calvin states "2 POINTS? IS THAT @#$% ALL?", which was just done to have Hobbes say "My, this game does teach new words!".