Verification builds:
1.2.5.15 Retail

Summary:
Having a layer of slabs over a 2 high gap will enable endermen to spawn in the 2 high gap, meaning that they immediately start to take suffocation damage.

Steps to Reproduce:

1. Travel to an area over the void in the end
2. Set difficulty to Peaceful
3. Place two layers of end stone with a gap of two
4. Place a layer of bottom-half slabs on top of the upper layer of end stone
5. Set difficulty to Hard
6. Wait for mobs to spawn on the lower end stone platform

Observed Results:
Endermen will start to spawn in o the lower layer, even though it is not a valid spawning point, and they start to suffocate.

Expected Results:
No mobs should spawn in, as there are no valid spawning platforms.

Screenshots/Videos attached: Yes

Notes: