I made an iron farm similar to Tangos HC8/VUAIF farm. Sometimes, an iron golem will spawn on top of a glass block or a sea lantern that is attached to slime and pushed by a piston.

Here is a replaymod recording showing the incident: https://youtu.be/rlaO–5BMvY

Here is showcase test world where I removed parts of the farm and added a few glass blocks to make the spawning more likely: https://www.mediafire.com/file/vljowdpl67pmg5i/Iron_Golem_Glass_Bug.7z/file
If you run the farm for a while (enable the clock in the middle) then eventually golems will spawn near the villagers on unspawnable blocks. It took me less than 30 seconds of carpet tick warp to see a golem spawn.