An invincible spider roams the world.

I was just walking around when I run into a spider. I pull out my diamond sword to kill it. I attack it twice, expecting that this spider would die. It actually looked as if it took damage and jumped at me. Over and over again, I attack. The spider still lives. I have no idea how this happened, but I suspect that it may have to do with the map. If so, here is the world generator seed, 1363916250.