Oh, it happened to me once!
The front prop fall down from transfer box, but not because I am deaf or asleep.
It was circlip that holds UJ's cup, poped out followed by the cup and then the prop. It happened so quickly Very, very unpleasant feeling...
The UJ has been changed 1 day ago in a workshop. Now, I'm not sure if it was workshop fault or rubbish circlip and probably I will never find out.
So, I think some kind of strap under the prop is very good idea.