Comic book publishing is an unusual art form, and in many ways, it structured its own rules as it matured. This was evident with the creation and adoption of the Comics Code of Authority in 1954, which Marvel and DC both adhered to voluntarily... at first.
Eventually, both publishers moved away from the CCA as the reader's interests matured alongside the artform. Depicting nudity, sexuality, drugs, alcoholism, and other subjects weren't considered taboo as they had been in the past. Once the CCA was rendered moot, there weren't many rules for comic book publishers to break.
On the other side of things, the ever-present independent comic book industry rarely adhered to any rules at all. Indie comics have long been the source for reader's interests in all manner of topics best left to their own dark and disturbing Wiki pages. Regardless, there's always one book that comes along to break a longstanding rule or tradition.
These ten comics and stories came along from various publishers and defied the rules. In some cases, there were repercussions, while in others, they merely changed the manner in which comic book stories were told thereafter. There are many more than these ten, though these are the most noteworthy.
10. Green Goblin Reborn! (1971)
Stan Lee was vocal in his annoyance at the strict rules imposed on creators by the CCA, and he was the first creator to challenge them. In The Amazing Spider-Man #96–98, a story called "Green Goblin Reborn" touched on drug addiction, which was a big no-no at the time.
The rules were so strict, a publisher couldn't get away with an anti-drug story, but Lee didn't care, and he wrote the books with Gil Kane on for illustrations and John Romita Sr. providing the inking. In the story, Peter moves in with Harry after accepting a job working for Norman Osborn.
At the time, Peter knew Osborn was the Green Goblin, but his amnesia mitigated that potential conflict. The story injects several instances of drug use, all of which were negative. The Green Goblin returns, but the real problem arose when Peter found out that Harry was popping pills to the point of suffering a drug overdose.
This was the first major story arc in the mainstream to buck the rules, and Marvel got away with it. In the end, "Green Goblin Reborn" was incredibly popular, and because all the drug use was negatively portrayed, nobody could object. Incidentally, the CCA seal was notably absent from these three issues.