While I was mapmaking I was creating a Zombie Boss mob with increased Health (200 Hitpoints/100 Hearts). When I was testing it I realized that the Zombie was unable to deal any damage to me even though he was hitting me, while normal Zombies could. After some research I found out that Zombies apparently deal more damage the lower their health is.
To fix this bug simply add an exception to the code where if a Zombie has more health than 20, it's treated as if it were 20.
How to reproduce:
Create a Zombie with extremely high Health value (in my case 200 instead of the normal 20) and let it hit you in Survival.
Attachments
Comments

This is untrue. The 1.5.0 version was modded, yes, but afterwards I checked the mobs in clean versions and it still existed. The spawner kept spawning mobs that only dealt damage after their helth was significantly decresased. This issue can be recreated without any mods, the only thing you need is an NBT editor (which my only installed mod was). Please reopen this issue again as it does in fact relate to a unmodded Minecraft, and while 3rd-party tools are needed to create the bug, this is intended (otherwise Dinnerbone would've never made it so that you can modify spawners after all...)

There is no feature "Zombies with unlimited health".
The game is not supposed to handle random values of random parameters edited with external tools.
Dinnerbone added the possibility to edit Spawners with external tools like NBTEdit to change all stats of any entity that is being spawned by these Spawners. One of the stats a mob, like a Zombie, has is the Health tag. By default this is 20, but it can be modified.
Now please tell me, if it was not intended that you can change these tags, why did he make it possible to spawn mods with modified tags?
Just for your information, these features were added in snapshot 12w26a, here's the wiki entry (there is no source listed, but I can confirm from experience, and I know for sure that Dinnerbone made a bunch of tweets about it):
"Monster Spawners now support extra information for the mobs it spawns. This allows unmodified games to have spawners for special variants of mobs. The player still requires external or modified means to set these parameters, though. Some examples of what Monster Spawners can spawn are Charged Creepers, Endermen holding specific blocks, Green Robe Villagers, and Villagers with custom trade offers."
If you still think this is unintended, please contact Dinnerbone to confirm, he will. Or ask a different mod here, but please one that knows the difference between "Modded game" and "Intended, but needs external tools to achieve"
How you're unaware of the probably hundreds of NBT tags that Dinnerbone has given people the ability to edit over the past months eludes me.
Dinnerbone has specifically given the ability to edit NBT tags for spawners, mobs, even world saves, mostly at the request of people such as Vechs and Sethbling.
If you don't believe me, try reading version changelogs on the wiki, or even putting in a single question to Dinnerbone – whom I would recommend you direct to this thread anyways, since, I don't think you're going to be able to help with a problem you don't think exists since you don't think this has been added.

This is a valid bug. However, the line between what is valid and what isn't is very thin and it's a blurry place when you involve 3rd party tools or mods.
Our main stance here is basically:
If you can reproduce it in completely vanilla, with no help from 3rd party tools or mods: it's definitely a bug.
If you can reproduce it in vanilla with no mods, but use 3rd party tools to get to it: it may or may not be a bug, it's extremely situational but almost every case is not a bug.
If you can only reproduce it with mods: It's not a bug to us.
Technically speaking, mobs with higher health than their max is a bug caused by a "health = max" line in our code that should be "this.health = max", but we've let it slide because it's cool functionality. That makes this even more tricky grounds to decide what's a bug and what isn't!
I'll fix this in 1.6, but there'll be a better way to do all of this 🙂
It'd be nice if we had a specific tagging function for issues relating to 3rd party tools and such, and for said issues like that, an extra box to label whether it's been tested in vanilla or not.
@Dinnerbone I noticed you changed it in the snapshots. So now the health tag simply doesn't do anything anymore. The mobs health is always the default spawn health. So, what's this "better way to do all of this"? Or do you still have to make that?
Well, it's sort of fixed in 13w21a. But it's actually even worse, now when you change the health. The zombie will always spawn with 20 health.
I can confirm this. I tried making a boss for may adventure map. It was a zombie with 200 health (100 hearts), but the attacks do no damage no matter its strength or weapon.
Edit:
I tested this later and it seems that anything over 40 health (20 hearts) will make it do no damage.

In 1.6 zombies no longer have this behaviour.
If zombies do more damage depending on their health and difficulty, then do zombies with more than 20 health deal less damage then normal?
> In 1.6 zombies no longer have this behaviour.
If so, please mark this issue as 'Work as Intended', not 'Fixed'.
@ANBO Motohiko
what's the difference between 'Work as Intended' and 'Fixed' then?
Works as Intended means that the "bug" is actually something that is supposed to happen. Fixed means that the "bug" was indeed a bug and has been resolved for a new version. The fact that this bug was fixed means than it was not intended.
Yeah, I thought so already. I don't see why ANBO Motohiko wants the to be resolved as 'works as intended'
This ticket is invalid as it relates to a modded or 3rd party client/server.
Any non-standard client/server build needs to be taken up with the appropriate team, not Mojang.
Any plugin issues need to be addressed to the plugin creator.
This site is for addressing issues related to the base unmodded MineCraft.