Do a search for a broken piece of porcelain from the spark plug falling into the spark plug hole. It happens. And I'm not sure what bike you have, but dirt and stuff does tend to accumulate around the spark plugs, in my experience. That's why they always tell you to blow the debris off with compressed or first lest the crap falls in. If sh!t falling into the cylinder were not a concern, then why is it standard procedure to clean the area off?
I'm not saying that that is the answer, but I was just throwing the idea out there. The last I checked, an answer hasn't been found yet.
Sorry for the input.
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