So I recently had a stuck spark plug in cylinder number 8. As you all know, that's a difficult one on the best days. Anyways, it was so stuck that the nut part broke off and I couldn't get an extraction tool in there. So, I removed the head. The small extraction tool wasn't cutting it so I bought a larger one.... This plug was so stuck that it's a permanent part of the head now. The extractor tool dug all the way to the other side and cracked my head. I've since bought a new head. I was told this happens because I didn't put anti seize in the plugs, however, the ngk recommendations say to install them dry, as anti seize may affect the torque settings (I never torque them, I just do hand tight and then a snug pull).
And yes, I soaked for 2 days in wd40, pb blaster, and an acetone/transmission fluid mixture. I also tried the female style extractor first.