Can carbon build up on only one spark plug?

Had a head gasket replaced on my 2001 nissan sentra....7 months later tried to change spark plugs and one had seized in the cylinder. It broke off, so I took the car to the mechanic that did the head job. They now want $1400.00 to get it out. They tried to say that the plug stuck because of carbon build up, but the other 3 plugs are just fine????