The recently added "game randomly adds doors to nearest village" mechanic (Mojang's attempt to grief iron golem spawners) could trigger the "adding a door to the blacksmith kills villagers" bug (the villagers see the wrong side of the door as "inside" due to the placement of the doorway and so get picked off by zombies at night). I've been unable to get the mods on this site to count that blacksmith thing as a bug (since a door would need to be manually added and they CLAIM mojang doesn't intend players to add to the village), but since this new mechanic in 1.8 causes the game to add the door it's now something that happens without player interference.

EDIT: Just to be clear the solution is to move the doorway so it's facing the street like every other village house. I'm not suggesting that the villager AI needs changing. It's the building that's the problem, and even then it's just where the doorway is that's the problem.