Been having this problem on my friends realm where creepers are spawning within his small lit house.  This has happened roughly 4 times and each time has resulted in everything getting destroyed.  The first time this had happened a player was afk within the home and the creeper had still spawned.  The house in question has torches on each wall and is not large enough for there to be gaps in light level, the house's internals are a 9x8 with a seperate room closed by a door that is a 9×5 area.  Unless torches where changed their light level when placed on each wall 3 blockd from the ground should sufficiently light up the house.