Last night, while I was pumping gas at a station near my house, I spotted a guy with his hood up. When I finished pumping, I pulled over to where he was and asked what was wrong with his car. He said he was just trying to put a spark plug in. I asked if it had popped out, and he said that he had. His car was a ~96 neon. He said that no matter how much he turned the plug, it never got tight in the hole (uh-oh). I took a look at the plug, and noticed two things:
1) the threads were chewed off the end of the plug
2) the gap was completely closed
I pointed out the threads and the gap, and asked if he had the old plug. He went digging through the car, and found it. It too had the gap completely closed, but the threads were intact. He also brought a new plug to try. I tried screwing in the plug by hand (using an extension and socket, but no ratchet), and it went about 1/4 of a turn and then stopped. I gave it a little twist with a ratchet, and it just came loose. It didn't grab again.
Since it was midnight and VERY cold out, I suggested that he unplug the fuel injector at that cylinder and drive home on 3cyls.
So anyway, clearly the guy had some problems with the threads in the head. He probably overtorqued or cross-threaded a plug, and messed up the threads. What I'm wondering is what happened to the gaps on those plugs - the gaps from both of the plugs that had been in the engine were completely closed, like someone had hit them with a hammer (only there was no damage that I could see). I have trouble imagining that they got hit by the piston, but I guess it's not impossible. Another wild chance is that some plug that he had in there previously is broken off halfway down the threads, and he was butting the new plugs up against the old one when he put them in.
Any other theories?