Im a tech a ford dealership, and have seen a quite a few of these, even on N/A motors.
I have yet to have one come in with plugs missing after doing a tune up, or after repairing one with missing threads.
I also am running a supercharger on my 99GT that uses the same heads as your bros 00L, and I have never had a problem blowing out a plug, and thats with more compression and 12psi boost.
What happens is the plugs works loose, and eventualy rattles the threads out of it.
The low amount of threads adds to a very small surface area for the threads to hold.
I recomend checking your plugs ever 10-15k, and screw the torque spec, but use comon sense when tightning them.
Yea I don't know what to say. I took the plug out. The plug has shavings on all of the threads. It also looks like all the threads were stripped rather that just a few of them. As if the entire plug shot out. But I find that hard to believe. I think your right in that it loosened up and rattled the threads away. Thanks for the input.