I'm not sure if this is actually a bug. Giving zombies or skeletons a helmet in the sun will take the damage that is supposed to be done to the mob and puts it on the helmet. Giving them a pumpkin will not damage it for obvious reasons. If you lose your armor it shouldn't be degraded by the time you get back to it.
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I think this is intended behavior. Helmets should prevent sun from reaching skeletons and zombies.
It's just weird because (I think) hitting a skeleton or zombie won't damage the armor it's wearing.
While I agree that a skeleton or zombie wearing a helmet should protect it from the sun and is working as intended, I can't say whether the helmet should take damage in the process while a pumpkin does not.
Yeah, the taking damage part is a bit weird, but I think that the sun causes damage to the mobs, which must either go into the helmet or the mob itself. I guess we should all carry a pumpkin around (just in case).

Yeah, the pumpkin cannot get damaged, and I don't think it's really possible to program damage values for a block, unless you want it to break in 16 hits.
@Anon Ymus
The mob will choose the best protection. If there is a leather helmet and a diamond helmet on the ground a skeleton will choose the diamond one or throw away the leather one if it picked it up. I'm assuming it works for pumpkins too.
Still, perhaps your good helmet falls off a cliff and the zombie only picks up the pumpkin.

works as intendent
Helmets also break when worn by zombie pigmen, who don't normally take damage from burning in the sun.
Still present in 13w22a

Is this still a concern in the current Minecraft version 1.6.4 / Launcher version 1.2.5 ? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

So a zombie or a skeleton wearing a helmet makes the helmet take damage from sunlight? How does that even work? It makes sense that the helmet protects the wearer from the sun, but it doesn't make any sense that the helmet would actually take damage from doing so, just because it's being worn by something that normally takes damage from sunlight.
I don't really mind the fact that it uses up the durability but the fact that it uses it up on zombie pigmen who don't burn is annoying.
still here 21w05b while i was curing a zombie villager helmet broke and it died

@unknown, as indicated in the report, this has been resolved as "Works As Intended". In other words, this is considered a feature, not a bug.
Confirmed. Gave a skeleton a leather helmet and set time to day. Waited for a couple of minutes and the helmet broke and the skeleton fried.
Whether or not its intended though I can't say.