Minecraft wiki entry for Things_not_to_do, associated text: If the player cures a zombie villager when other zombies are around, the other zombies attack the cured villager, reversing what the player just did. Even if the cured zombie is safe from zombie attacks, it takes longer to be cured, so zombie villagers should be kept away from zombies as they are being cured. Also, this wastes golden apples and potions of weakness, losing gold and other valuable materials.