In the 12w07a snapshot of Minecraft 1.2.1 Zombie sieges were added. They occur in the night, with a 10% chance of happening. For more information, visit this site: http://minecraft.gamepedia.com/Zombie_siege

Anyway, the problem is that I've noticed multiple times, that the zombies in sieges spawn in some awkward places. For example, even though I didn't take any screenshot, once a Zombie spawned on top of a glass block, and another time all of the zombies spawned inside of nether brick slabs.

All of the zombie sieges mentioned on this report happened in/on either a Villager Farm or an Iron farm.