I had been reading the comic book The Sandman regularly from the first issue and was getting to be a big fan of the series, until one about issue 6 or so. It was about a bunch of people who were trapped in a diner with a supernatural mass murderer (was that The Corinthian? Or did he come later?). It described how he would control the minds of the customers to make them do sexual and violent things to themselves and each other, eventually killing everyone in the diner. It was pretty graphic and disturbing, but the part that got me the worst was a single panel where he said something like “for one hour, he gave the people back their minds and made them realize everything that they had done.”
That whole issue really got to me, and it turned me off the series for about a year. I eventually picked up another issue, liked it a lot and saw that that wasn’t the tone for the entire series, so I started reading it again. And I had to go back and spend a fortune on back-issues to get all the ones I’d missed. (It had gotten insanely popular in the meantime.)
Another one from comics:
There’s an issue of Preacher by Garth Ennis, where one of the characters is talking about the plane flight from Dallas after JFK was assassinated. He tells the story of going into the cargo hold of the plane, right after LBJ was sworn in, to find LBJ fucking the bullet hole in the president’s skull, screaming “Who’s in charge now, huh? Who’s in charge now?!?”
That was about the most obscene thing I’d ever seen, and it turned me off the rest of the series. I mean, I think I “get” black humor and exaggeration, but that just crossed the line for me.