Alot of the 5.4 Triton engines (maybe others as well) had issues with the spark plugs. The metal fails and the plug either strips or removes a chunk of cylinder head metal. Don't mean to say this is common, but there is a Ford Factory repair kit for it (time-cert)... Our local mechanics keep them in stock. They are even known to blowout of cylinder head by themselves. Changing the plugs is not a happy-happy joy-joy event either as with each turn of the spark plug took you are wondering if it's going to come out OK or come out very very badly.
This is not to say this is a bad vehicle - far from it, this is a good, smooth running, strong engine. But it is a very widely known problem.
Spark Plug Ejected From Engine - Ford Problems.com
angela