I have seen the cermic center of the plug blow out and leave the metal threads in the head. The engine in queston was an Atomic 4 with probably 20 year old heavly rusted plugs. Eventually the rust got them and pop.

I'd replace all the plugs in that engine asap.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200