I did a quick search and nothing came up, SO... (TL;DR skip to Edit2)

I woke up one morning in MC and went outside to find a fully armored Villager Zombie. I thought "oh great, I can cure him. he ain't gonna die anytime soon."
So, I led him into my house where I attempted to trap him, which didnt work out, so my wolf got up and started attacking him, after hurting me. I quickly made my wolf sit, and the zombie stood in the corner. Even after walking around out of the corner he didn't attack me. He did slip out of my house and I managed to push him back in again (pain in the arse, because even when I punched him, he stood there neutrally). After trapping him successfully (cause I could push him around like a passive mob), I saved and exited and came back. He was still neutral until I punched him.

I'm not sure if this bug could be replicated too often, but here's a picture of him not trying to kill me in point-blank range.

Edit: Supplied a crash log, sorry for any inconvenience previously.

Edit2: I CAN REPRODUCE IT! Im so happy!

Steps to reproduce:
1-Spawn a zombie, and a wolf, Tame the wolf.
2-Hold a sword and go into survival mode, let the zombie hit you so that the wolf attacks teh zombie.
3-While still holding the sword, right-click your tamed wolf (a few times).
4-The wolf will sit (probably will be standing visually for a second) and the fighting will stop.
5-The zombie will lose aggression towards you.
6-If the wolf dies, the zombie gets aggro on you again.
7-You can now command your dog to follow you and the zombie will stay friendly.

NOTES: I think i actually used the fence post in the original bug, as shown by the photo, but the sword works too. I managed to have 5 neutral zombies at one time, before my wolf died and all hell broke loose (on a test map thankfully). I realised after more testing it doesnt ALWAYS work, as in some cases the zombie will go passive and teh wolf will continue to attack, or vice versa.

Edit3 I completely forgot about this bug report, thinking it had been patched already. What's it been...a little less than a year? Glad people pay more attention to the tracker than I do.

http://youtu.be/LKmpUAWtytU

Video demonstration.

Edit4 After looking at the video, I could probably tell you why this bug occurs:

-Zombies only have aggression towards one target
-While the wolf attacks the zombie, the wolf takes aggro
-When tricking the wolf into sitting, it stops attacking
-The zombie still has aggro for the wolf, but can't attack, resulting in stalemate.

Edit5 Still in 1.8.2, however after testing I noticed that if you punch a friendly zombie, your wolf will start attacking it, maybe not right away, but it will.