Mob spawners utilizing the "SpawnPotentials" property create ghost entities every time they fail to spawn. This causes the the entity count to increase even if no mobs are being spawned (see the debug menu), and eventually causes a crash due to thousands of entities. These ghost entities cannot be seen and cannot be removed with /kill.

This problem started with 14w03b and is still occurring in 14w06b. It seems to be client-side because logging out and back in (SSP or SMP) clears the entity count back to normal.

Below are a few commands useful for observing the bug.

setblock ~0 ~1 ~0 mob_spawner 0 replace {SpawnPotentials:[{Type:"Villager",Weight:1,}],SpawnCount:1,SpawnRange:10,Delay:-1,MinSpawnDelay:10,MaxSpawnDelay:10,MaxNearbyEntities:1,RequiredPlayerRange:20}

setblock ~0 ~1 ~0 mob_spawner 0 replace {EntityId:"Villager",SpawnCount:1,SpawnRange:10,Delay:-1,MinSpawnDelay:10,MaxSpawnDelay:10,MaxNearbyEntities:1,RequiredPlayerRange:20}