Yeah, this hasn't happened, but I could see this happening at some point:
What is the best thing to do if or when someone has has been triggered by a traumatic experience during the session (in the instance that no physical boundary has been breached)? E.g. touching the shoulder, or talking about a memory, or having a song or position trigger a previous experience, etc.
My initial thoughts are to take a break or end the session abruptly - maybe ask if they'd like to talk about it or need a moment or space.
Humans are sensitive creatures, and no one is immune from "having a moment."