Monsters Spawning in lighted areas.

Monsters on my multiplayer server will spawn anywhere. Regardless of whether or not the area is lighted. This map was started on a "Snapshot" version and I suspect that the bug has to do with the fact that it was played in a different version. I experience extreme lag after I was attacked by a zombie inside my house. There is NO way he could have spawned. Therefor something must be wrong.