What I expected to happen was...:
It was my understanding that no matter the light level, there were still certain blocks that hostile mobs couldn't spawn on. (IE Light source blocks {glow stone sea lantern}, Stairs, Lower half Slabs, carpet, trapdoors, ect)
What actually happened was...:
Hostile Mobs (creepers specifically) Were spawning on rooftops that only had stairs, carpet, lower half slabs.
Steps to Reproduce:
1. After claiming a Village I assembled some villager homes and added carpets to all roof blocks (Excluding lower half slabs and stairs.)
2. At night Creepers have spawned on rooftops with carpets stairs and lower half slabs.
3. Still unknown if any other hostile mobs spawn there or not, so far only creepers have been confirmed.
Could you post a screenshot of the roof the creeper spawned on?