Well, it finally happened to me....the dreaded cross thread. I removed my Mercury plugs to inspect and clean and upon reinstallation, I had an issue with the last plug. I started it by hand like all the others and it went in smooth. Once I started easing it on in with plug socket, it went ok for a few rounds and then just stopped. I bought a thread chaser at local Auto Zone and used my ratchet driver to ease in and clean up threads. I did apply grease to the thread chaser to collect any metal shavings. The spark plug went in just fine afterwards. Not sure if it was carbon in thread grooves when I removed the plug or what but all is good now.

You probably didn't cross thread the plug. It probably tried to seize up because of either carbon in the threads or maybe the dissimilar metals. I worked for VW for several years and we had the same problem nearly every day. Lots of time we couldn't get the plugs out because they were put in dry. You need some kind of anti-seize on the plug threads. You can get Fel-pro anti-seize at most auto parts stores and it works great. In a pinch graphite will work also. Just remember Aluminum and steel don't always mix and Aluminum always loses.

Johnny