The needle example is an interesting one. I think if a student declared that they had a phobia of x at some point, I would absolutely warn them that x will be shown in class/present in a text on a particular week. But it's difficult to account for all phobias. What if there was a movie with a spider in it? Or a book? Should the professor warn people with arachnophobia to leave/shut their eyes at a particular point, or skip a particular chapter of the book? _refugee_ mentioned being warned about death being discussed in a text. I've seen course syllabi where every single week could be marked with "Trigger warning: death". It's really difficult to predict how people will react to certain texts, and trying to put in trigger warnings can quickly become a strange guessing game of figuring out (and sometimes spoiling) challenging aspects of texts.