I've ran into this problem several times now through out the pre-releases. The shield becomes permanently active - slowing down the player, making them block almost all damage, and making them immune to knockback effects.

Taking the shield off doesn't seem to fix the issue either. Reloading the Minecraft world can fix it though.

This bug basically makes the player immortal and allows them to keep swinging with a sword and using other items.

Currently I don't know how to recreate the bug, all I know is it's happened to me a good 15-20 times now.

Edit: I found a way to partially