Long time reader here. I just had to pass this along cause it's a good one:

The old brick starts running rough at idle a few days ago and on comes the "check engine" light. Seems to run fine at cruise. Along with this is a rich smell from the exhaust and reduced fuel economy. It came on suddenly. So I dump the codes and they are all over the place: 1-1-3, Fuel trim: too rich or lean; 2-2-1, fuel trim: too lean at part load; 2-3-2, fuel trim: too rich or too lean at idle. Of course I think the worst: bad AMM or 02 sensor, even though there are no codes set for them. Thinking it may be a bad plug or injector I decide to try and isolate the possible bad cylinder. Pull off first plug boot(front): no change. Second: same. Grab third plug boot and it flops into my hand on with no pulling. OK....
Try and push it back on: all slop. Look in boot (thinking maybe the connector is corroded, broken, etc. Hmmmmmm: it doesn't look like a metal circle. Look at plug in the head: see threads where the top knob of the plug usually is! The knob had completely backed off the plug! Twist boot enough to get it started, pulled boot again and tightened down the knob. Voila!! Smooth idle. Checked other three: all are loose too: at least one turn. Pretty certain the issue is resolved. Cleared codes. We'll see. Lesson: check all the little things first.

I have been wrenching since I was 16. 40+ years. Never ran across this before nor checked for it.