Cylinder #3 bottom plug

Fellow cubbers,

I had the bottom plug on number three go "bad" 60 hours after an annual....Total time on brand new engine 400 hours. Mech says the plug must have just gone bad. This plug was PLUGGED up with black slightly greasy pretty dry but not candied on deposits that filled it up.
It was a real slow increase in the size of the mag drop until it topped the 150 RPM drop and I started pulling things apart. If I leaned it out I could usually bring the drop to less that 150.

I do hand prop and I did leave it on L for alot of ground taxi and went to run up then did not leave ground until found the plug.

I have never had a plug do this and was wondering if there is anything I am missing out on checking?

Put in a brand new store bought plug and it drops less that 50 per side now.

I run 100LL and don't lean it out at cruise but do when I am on the ground.

I'd watch it real good and try to remember to switch to both. I lean all the time except climb to cut done on lead build up in the sparkplugs. I have a friend that flies pipeline and in the C152 it will have leadcoming out of the bottom plugs on the bottom after 100 hrs. If you have ecessive oil use in that cylinder it will show up this way but is usually wet not dry as you described.