oil in the spark plug
I had the same problem when I changed my plugs. Hasn't caused any problems, just messy when you pull out the old one and insert the new one. Isn't on every plug either. I always thought it was probably the valve cover gasket. I had a Chevy Beretta with a V6 that was the same way. Who ever thought of the idea of putting those darn plugs down so deep anyways.