To add to my comment, in a convention I run, a player was triggered in one of the games. I’m talking about something apparently minor in the fiction. It was dealt with at the table by taking a break and changing direction. I wasn’t alerted as organiser, and didn’t find out until afterwards; everyone was happy with the outcome.

I’m not sure what my point is, except to say this sort of thing does happen.

On the X-card, the time when I’d want it is if we were potentially pushing for dark and uncomfortable content (beyond the “typical” RPG action adventure fare), and we need an option to quickly veto something personally awful. As such, it would be an enabler to explore dark themes; I wouldn’t want to go into that without an X-card or something similar, and lines and veils wouldn’t really work.

Paul Beakley​’s example of an X-card against a particular word choice (even for much worse word choices) probably wouldn’t come up in the UK.