Right then:

Kawasaki ZX6R 2001; riding along on the motorway 80-ish in 6th gear, starts spluttering as if it needs to go on reserve although that'snot the case.
Loses power and runs on three pots and smokes quite badly.
No rattles or any other noises.

**Yes, we would stop asap but in this case the clueless tit rides it home and no it wasn't me **

I took the airbox off to fins oil inside as well as aluminium debris so I guessed it dropped a piston...and it did on no.2

The sparkplug was rather reluctant to come out and when it did it turned out to be smashed and was covered in debris, aluminium shavings were actually coming out of the no.2 exhaustport

So what could have caused this to happen on 20.000 km bike wich has been very well maintained and ridden rather sensibly (apart from riding after the smokey bit )?

Sparkplugs were the correct type, oil was fresh and up to correct level, carbs are clean, floatbowls full of petrol and mainjets clean as well?