Day one pit iron farm

A cheap early game iron farm for three villagers and one zombie, based on ianxofour's Day 1 design. The villagers sleep above a pit, the zombie panics them from below, and the golems fall into lava over a single hopper. No redstone and almost no iron to start it, so it is buildable on the first day. Java only.

Golems spawn anywhere in a 17x17 area around the beds, not only in the pit. Slab or path the whole area and fill every cave and hole underneath it. This is by far the most common reason the farm looks broken.
Let the villagers sleep a full night and claim their beds before you bring the zombie down. A villager that never claimed a bed does not spawn golems.
The villagers have to keep sleeping or the farm stops. Swap the boat for a minecart on powered rails, or a piston that moves the zombie in and out of sight, or put two blocks on the slabs beside the villagers so they can break line of sight at night.
Place a solid block under the slabs and under the trapdoor in front of the zombie.
Build at least 100 blocks away from the nearest village.
Java only. Several people spent hours on it before noticing.