I know the title is no surprise but it's an interesting story.

I developed (quite suddenly) a shudder at idle. Only like sitting at a stop light. It got worse quickly, and then I got the check engine light.
Pulled the codes, and it was the cylinder 3 misfire. So I swapped coil wires 3&4 to rule out the coil and the code stayed in cylinder 3 so I started pulling plugs.

Old engine, I was a bit nervous, thinking a bad valve maybe, so since I had plugs out I pulled the fuel pump fuse and yanked the power to the coil pack and did a compression test - it was fine.

I had all the plugs lined up on the workbench and my son was like "Why is this one different?" and I looked and sure enough, the plug from cylinder 3 was a motorcraft, and all the rest were autolite.

It didn't look terribly *bad* though, it definitely needed replacing but I've seen worse.

Then I noticed the nut on it was almost rounded. I didn't have any trouble getting it out though. But that plug does require a flex head wrench to get it out, and suddenly I figured out what happened.

Someone tried to replace all the plugs, couldn't get that number 3 out, and basically gave up and just replaced the other 5. So I think that old motorcraft plug basically just died of old age lol

It is interesting though - the symptom of rough idle, but it ran fine once you hit the gas and started moving. (I mean, fine for an old slow-*** 3.0 anyway)

Kinda fun story thought I would share.