lost a sparkplug

Today my wifes 2002 2.0 n/a forester spitted out a sparplug.
It has only 180.000km on the odo and was running excellent before this happened.

Try to put in a new plug but there is no screw-thread left

Its left side (european drivers side) nearby the headlights. Is it possible to repair this with a helicoil or do I have to pull the cillinderhead?

I'm puzzeled why this has happened. The sparkplugs are in a 30.000km. Any ideas?

No idea, never heard this happen before. You have no luck with your Subarus...

^ me either! That's bad luck!

Oh no, Casper ...

Probable cause would have to be that the plug has been over-tightened at some time.

Should be able to have it helicoiled, but you might have to remove the head .
Get advice before you dismantle it though.

they can be done in the head on the SOHC engines as the plug pokes up at an accessible angle.
a thread repair specialist should be your first call to discuss

Nasty! Did you forget to wear the red sock on your left foot or something? What a thing to happen.