OK, my 99 F150 has given me fits again. I have the 5.4 with around 100,000 miles on this engine.
A few weeks ago the check engine light came on. It didn't run bad or anything. Then last weekend driving down the highway it started missing. I checked and found the second plug from the front on the drivers side was not firing. The plug had been smashed, no gap. When the miss started it had a loud hard knock that went away after a minute or two. The plug was beat up, but the porcelen was not obviously damaged. I re set the gap and put the plug back in, it runs better. It has a slight shudder, not really a miss just idles a touch rough and especially when it is cold has lost power. Step on the gas it falls flat, but does not die.
What could have broken loose and fell into the cylinder to smash the plug? Anyone else have this happen?

 

I have seen the weld come loose on the electrode and smash into the plug. Was the electrode still sound on the base of the plug and just bent down tight? I had a JD tractor once that stopped running. Pulled the plug out and the electrode was gone and the base looked beat up....I assume from arcing...????...not sure. A new plug and all was good. Never did figure out why it did it.

 

The electrode was solid. Looked like something small had beat it up, but the plug was good after I reset the gap.
Wondering if there is a sensor that could have broke off in the intake and ran through, or a small screw that could have broke off and been pulled through the valve into the cylinder.

 

But what are the odds it would get directly under the plug? Maybe I suppose, how much room is there when the intake valve is all the way open...i.e.what could fit?

 

That is what I have been trying to figure out. I have seen nuts and screws slip past the valves on other engines and mess things up, but usually that embeds in a piston and breaks the piston. At 2500 rpms it would only have to contact the plug a couple of times.

 

run a compression test and if the cylinder is good, install a new plug ... COP if needed.