It was a common myth many many years ago, but have never heard of one comming out enough to hurt any thing. More prevelent with open (difs) of failure since these are stock low hp units with a lot of parts and gears involved, but with a spool the ONLY thing that will happen is that you will knock the teeth off.
The main point of injury in those times is when all that stock stuff broke at speed, it would lock the whole rear end and the rear axle housing would try and spin out of the chassie and that has nailed a few drivers, Hence we now are required to have 4 bolts per side on the rear housing to mount to frame and anti rotation plates or struts to keep that from happening.
To me the bigger issue is people trying to use the stock Ford case and pinion support instead of and aftermarket unit, they are readely available and the priceing has come was down. I would be more worried of the pinion being pushed out the front of the case then the gears comming out the back
