I have a bunch of doors that are all in a huge group all facing the same direction, and mobs spawn in it. Mobs are spawning in the spaces between the doors.
Also, it seems to happen only when the doors are under a light glitch where it acts like its not the brightness that it is, but I think there was a spot where a mob spawned where it was NOT under a light glitch. I think.
Also, the mobs that have appeared are zombies, creepers, and skeletons.
I think a Witch too .....
Mobs will spawn if there is space for them to spawn and not enough light to prevent them from spawning.