The one time I saw it happen a live round got loaded in a mag full of blanks. All it did was fire the metal plug thingy 25 yards down range. The weapon was still fully functional and undamaged.
No one could figure out how it happened and no one was injured but after that I insisted that only “I” load my magazines and in practice exercises I never aim directly at anybody.
The guy who fired it felt really bad and insisted that he did not load the magazine with the live round which was probably true but it would have been of no comfort if someone was injured and he knew it.
Thanks for the first hand report, I never saw one happen!