I agree they are beginners, and using a safe word is a good thing, but to tout it as a necessity like others have implied is a bit much.
As far as live and learn... yea agree there. I nearly dislocated my submissives jaw once while face slapping. She didn't use her safe word, it was just a change in her muscles, and look in her eyes that gave the "I think you dislocated or broke my jaw" kind of look that suggested I stop. She was fine after about 3 days of jaw exercises to keep things limber, up till then talking, eating and moving the jaw around was taken slow.