Several Lightnings(5.4L) have had plugs that blew out. Others have found the plugs little more than finger tight.
When you go to change them, remember to not over tighten them as there are only 4 or 5 threads to hold them in the aluminum head. On the Ls, you can't get a torque wrench(13 ft. lbs.) on several plugs so NGK says to tightne them "finger-tight" and then 1/16th of a turn more--careful!
i have seen this alot. and from what i understand. the head can not be fixed. to last any time. install new head. is the fix i have seen. alot of the early 5.4's had this concern. the newer one's are ok. not sure about the 4.6.
I had this happen on my expedition with 38,000 miles on it. I was at idle when it broke a chunk off the head. Like to never got it to the dealer. They were pretty amazed, they had never seen it before.