Hostile mobs can spawn on bottom half slabs, stairs, and fences. In addition, they can spawn inside leaves if the block below is spawnable. I believe they can spawn on glass. None of these blocks should be spawnable unless (in the case of slabs and stairs) unless they are oriented so that a full block face is on the top, as in top slabs and upside down stairs.

Spawn rules are clearly documented in the wiki, and MCPE definitely does not conform to PCs behavior. It is important to have alternatives to lighting for spawn control, and the ability to make safe roofs without odd roof lighting.