I was exploring on one of the snapshots for 1.8, under the customized world with dungeons at 100, and in a regular dungeon, mobs would spawn inside of the walls and suffocate.

When I saw mobs spawn, they should have spawned regularly.

Instead, they spawned in the walls and suffocated.

1. Find a dungeon.
2. See that some mobs spawn in the walls, others can spawn regularly.

This can take a while to happen.