So, Minecraft.

[kandrathe]

I upgraded my little dirtshack to a much larger A-Frame thing, but I was stupid and didn't realise that it needed more light. Offending monsters spawned within. http://imgur.com/VuUwB

Odd thing about monsters spawning in houses... There is a mythology about it. If you build a 3x3 1 story house and place a bed in it, invariably you will awaken to a mob in your house - regardless of light levels. Some people blame dirt floors, or blocks not meeting squarely, but that doesn't seem to be it. I've had large houses with the same issue though. I tend to always sleep in an upper story room now. The first floor is too spooky.

A trick... They can't spawn on top of glass blocks. So, any dark horizontal surface bigger than 2x2 will allow them to spawn.