"this is clearly marked so that the user won't do it."

No - of course they won't... :-0

Just like this forum tells everyone to do a 'Search' before posting and, of course, they all do... ;-)

My current client makes a device to be connected to the phone line; I have seen several burnt & charred examples where users have connected the phone terminals to the 240VAC mains!!!!!! (there should be no connection at all to the mains!)

No, if there is any possibility to do that which should not be done, you can be absolutely sure that some user will do it - no matter how many dire warnings you give! :-(