Steps to reproduce:
1. Make sure you're difficulty is set on 'Hard'.
2. Create or enter an existing world (preferably on 'Creative').
3. Build a small house and place a wooden door on the entrance.
4. Wait until night or "/time set night", then spawn one or more zombies in front of your small house.
5. Enter your house and either a) switch to survival or b) spawn one or more villagers inside.

Zombies will start piling in front of the door but none will make an attempt to break it down.

First noticed this issue in 1.7.2 but it seems to exist back to snapshot 13w42a.