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.