I run into this a few times, and it's part of the reason why for my longer stories over at RavishU, I'll put codes for each chapter. I'll have a general disclaimer at the top of the story and with some shorter ones I might just have one series of tags at the beginning. But there's times I'll have to go back and recode them and redo the disclaimer.
For example, I have a story called Lake Hiawatha. When I wrote the codes, I didn't know how fucked up I was going to make one chapter, I just knew it was going to be pretty fucked up. 500 screenplay pages later, and I wrote the most fucked-up sex scene I've ever written. Cannibalism was not one of the original tags I planned for in the story, and there's a couple things in that scene I'm not sure how to tag, like 'is it necrophilia if they're killed while fucking the antagonist?' and 'spraying someone with the remains of their boyfriend going through a woodchopper, is that watersports, gore, or both?' After writing the chapter, I had to stop, take several deep breaths, take a shower to wash off the filth, then go back and retag that section of the story and put some warnings in it.
I try to give warning ahead of time, and if something comes up once or twice I'll use the tags for the chapter. But if it becomes a recurring theme, I'll put it in the disclaimer to start the story. I also like to have fun with the disclaimer. I will put a disclaimer for things like racism, bigotry, unhealthy behaviors, excessive drinking or drug use, going to church, smoking, and veganism. "The author does not condone any of those activities, except for the occasional veganism. It wouldn't kill you to eat a god damned salad once in a while, and no, Taco Salad does not count."