When using a custom mob spawner (I haven't test with a normal one) which should spawn only one entity (villager for me), the entity cap is dramatically increasing by 1 new "ghost" per second (any entity is visible or interactive in any way in the game and does not appear using external tool such as MCEdit). More, the entity aren't blocked to the 90 mobs limit and it can raise easily thousand of ghost entities if not noticed, though the game is lagging after a few hundreds.

To reproduce:

• Open a new map.
• Use MCEdit to put the schematic linked lower on the map.
• In-game, check the entity cap raising.

I tested that on two different computers.