I believe this well-known manufacturer defect affects the following 2-valve engines: Triton 4.6L V8, 5.4L V8, 6.8L V10 & possibly others.
In TSB 07-21-2, Ford acknowledges that "some 1997-2008 vehicles equipped with a 4.6L 2V, 5.4L 2V, or 6.8L 2V engine and aluminum heads may experience a spark plug port with stripped or missing threads".
When a plug is loose and an engine is rotating at 2000 rpm, it generates a tremendous amount of pressure each time it fires a cylinder.
So the longer a spark plug wobbles from being loose in the threads the more thread damage it does and finally the last few good threads can no longer take the pressure and out comes the plug, taking the last few threads with it.
So here we are and what do we do now? Most troubling to me is the huge diversity of dollar cost and repair procedures for a relatively simple thing to fix. A thirty dollar HeliCoil kit can do the trick if installed correctly with a good loctite compound installed on the threads of the repair insert. Sadly most mechanics just re-tap the hole and throw an insert in and a new plug. It usually lasts a while and then blows again creating a bigger problem! Don't panic! IT CAN BE FIXED!
The best repair is to remove the cylinder head and replace - but then you are only going back to the original poor design (lack of threads to secure the spark plug in the first place).