In my case the plugs look like I installed them last week. They all look the same - clean. After further inconclusive testing, I think heard a tick-tick-tick from the area of the one bad plug suggesting a bad insulator or cracked ceramic case or something like that.
I haven't sworn off Bosch Supers, but I have to wait until I have to put in a worthwhile internet order since I can't find a local shop that stocks H9DCOs. 'til then it's cheapo Autolite 104s. But there might not be a 'til then since I'll be selling the 300SE shortly.