An iron golem farm constructed such that all horizontal blocks outside the desired spawn zone are covered in half slabs andor glass, is not infrequently spawning golems on top of the half slabs. Note this is slightly different behavior from
MC-3927, where a golem is spawned inside a half-slab and gets stuck. These golems are on top, can move around freely, and are vastly annoying. It doesn't happen often – maybe one every 2 hours or so. But that's enough to break the whole farm in under a day.
The problem is not new to 1.8.1. I've been facing this since I built the farm back in September.
The first three attached pictures attached show the golem spawned on the half slabs, including one close-up with F3 enabled showing the ground on which it stands, and one further back showing the overall structure (if it helps any). Also included is a shot of the upper deck of the collector which also spawned me a golem this morning (didn't get the pic in time before it walked off the edge. Like I said, they're not stuck in these slabs; at least I managed to solve for that problem) Update: two additional shots showing where a golem spawned on half-slabs in the upper deck village.
My map is available on request if that would help the defect team at all.