The Bug
2day I was trying to cure a Zombie Villager near a group of Villagers. I was about curing whom, while a wild Iron Golem appeared all of a sudden IN MY FACE, and, of course, who killed that Zombie Villager.
How to Produce?
Summon which can attack Villagers nearby, wait 4 a while.
PS
That really made me confused when a telegraph pole (XD) suddenly appears in my face.
When 3+ villagers detect a zombie nearby and get frightened, they spawn an iron golem, provided they meet the correct criteria of slept and worked. I don't believe this is a bug, just bad luck.