I have been experiencing this since 1.3.2 I believe. I've had my mob grinders turned off since then. Tried them again with the update the 1.3.4 with the same result. Occasionally mobs will spawn with no hp information/infinite health. Only way I have found to kill them is to either safari net them and throw them in the ocean to despawn or use Blood Magic sacrificial blade on them. This does not produce any blood as far as I have been able to tell. Safari net calls the mob "unknown"
Happens with creepers, tele-creepers, endermen, blazes, skeletons and zombies so far.