Skeletons and Zombies do not burn when on a "block" that is under the height of a block, such as slabs and stairs.
What I expected to happen was...:
That the mobs would burn.
What actually happened was...:
Skeletons walked on to a slab and stopped burning. I then tested this with a zombie as well.
Steps to Reproduce:
1. Make a slab on any surface.
2. Spawn a 'burnable' mob on said slab.
3. See as they do not burn.