I use back-tick `. For nested, it becomes `` or ````. For twice nested and I'm writing a shell script with ` instead of $( .. ) It's ```````` (I'm an emacs user, I need all the control characters :)
Even without tmux, this can be an issue - a long time ago I was using the X windows "drag the selection" middle mouse button action and accidentally let go of the button while the cursor was over an emacs text window - I sent my first email to some big listserv I subscribed to consisting of weird fragments from the log file I was trying to paste.a