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Also happens with iron bars, but it may be due to the Zombies not being able to hit their hit-box. If I try to attack a Villager in a window/iron bars, I can't hit them either.
It happens with fences, too.
@Joss Eden:
When I tested it, I was able to hit the villagers. So were the zombies, but they would just stare at the villagers for a while and sometimes began/resumed attacking. That means they can hit the villager's hitbox, but the zombies just think they can't (probably because they think the fences/glass panes/iron bars are full blocks).
Other affected versions are 1.5.2 and presumably 1.5.1.
Here's some more information from my duplicate issue (MC-18610):
Sometimes if the villager moves or is bumped, the zombie would start attacking again. Strangely, if you hit the villager then bump the zombie, the zombie will attack again. Sometimes the zombie would start attacking again on its own, but that was rare.
About the new title: It's not just the corners of fences, but whenever the villager is up against a fence (doesn't have to be a corner).
I have some screenshots of this.

Is this still a concern in the current Minecraft version 1.8.1 Prerelease 3 / Launcher version 1.5.3 or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
No response for over a year.