Very rarely zombies spawn in a mushroom biome.

I have an enclosed dark area, 5x5 large. There is no way in or out unless I break a block. It is in the center of the mushroom biome, and I've checked every space with F3 to check if any blocks were not considered mushroom_fields. A Zombie should not be able to spawn in there. 

It does not happen often. This has happened only twice in my time playing with the new snapshots. So my guess would be that this happens maybe once every 50-70 hours of gameplay. 

It is a survival world.