Sascha Baer

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"

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...)

This is an annoying one, you can only use /kill on yourself. Most likely intended, but shouldn't be like that imo.

Since lighting got rewritten for 13w07a, this bug MIGHT be fixed. However I can not acess the snapshot downloat at the moment so I can't check if that's the case. Can someone who has acess to the snapshot test it real quick?
Edit: okay mojang.com is back up and they didn't include the fixed lighting. That means the bug still exists in 13w07a

Last pic was by me. It was taken on Vechs' Super Hostile #14 - Waking up, in Helix Cavern.

@Markku: Normal Superflat preset, make sure the area you incase has a whole chunk in it (like, from -5/-5 to 20/20). I just tested it, the bug's still there (obviously, it wasn't fixed)

@Christian: mmh... restarting the game usually reloads the chunks, but if that's the case I'll add that into the description

Very bad bug description...
What happened before the world got corrupted? Did your PC crash? Or you closed your minecraft wihout saving (CAN potentually corrupt your game)