There are special versions of Spawners that utilize the spawn-once feature of mismatched SpawnData and SpawnPotentials tags. The SpawnData uses proper coordinates, and the SpawnPotentials uses void coordinates. Control of these spawners is done using spawner carts and lava. I have discovered an instance where a base spawner responsible for spawning spawner carts is creating a massive number of phantom item entities when they are stacked. I think these item entities are being created for display on a spawner cart, but end up at the spawner cart's spawn location (not the actual location of the spawner cart). Since these tags are deeply nested in two ways, it is difficult to explain. Attached is an MCEdit schematic of a spawner that reproduces this problem. The stacked items it is spawning are indeed an extreme example, but I've had similar problems with a reduced number of stacked items. Single items spawned using this system of spawners/cart spawners do not exhibit this behavior.