9. Jennika Is Reborn - Teenage Mutant Ninja Turtles (2019) #95
When Jennika was initially introduced to the TMNT comics, there was a lot that was different about her - her attitude, her personality, her beliefs - oh, and that she wasn't a mutated turtle woman.
But, in TMNT #95, this all changed, when Jennika was left in critical condition after an attack from the Foot. Aware her only chance of survival was a blood transfusion, Leonardo gave his turtle ichor to save Jennika's life.
Though this worked, it also resulted in the former assassin becoming mutated into turtle form, which is traumatic for everyone involved, including the reader. It's also shown in some straight-up horror level detail, presumably under the hope that you'd buy further issues of the comic so you see the turtles somewhere other than your nightmares every single night.
This said, Jennika does become a super interesting member of the team, so the trauma was kind of worth it in the end.