When spawning and killing lots of mobs, memory usage will rise and will not go down even though all the mobs that were spawned were killed. Memory usage will stay high even when saving and exiting the world and loading another world.

What I expected to happen was...:
Memory usage wouldn't be so high

What actually happened was...:
Memory usage got so high that Minecraft started to freeze for small periods of time which then eventually caused Minecraft to run out of memory.

Steps to Reproduce:
It's easier to do this in a medium to large room with obsidian walls, floors and ceiling.

1. Spawn lots of skeletons
2. Spawn lots of witches
3. Have skeletons attack each other and witches attack each other by going into survival mode
4. Spawn creepers to kill the witches and skeletons
5. When amount of skeletons remaining gets to a low level, spawn more skeletons
6. When amount of witches remaining gets to a low level, spawn more witches
7. Keep repeating steps 4 to 6 until you notice the game starting to freeze for small periods of time every few frames or when the screen appears saying Minecraft ran out of memory