The static initializer for PrimitiveMobsParticles caches a reference to the current WorldClient which is never released, leaking memory. Steps to reproduce:

1. create a single-player world
2. spawn a TreasureSlime to make sure the static initializer is called (I think - my Java ain't too good!)
3. quit to main menu
4. reload the save from step 1

Examining the heap e.g. with VisualVM shows two WorldClient objects, the second of which is referenced by the Minecraft object and the first by PrimitiveMobParticles.theWorld.

Nice catch thats quite old stuff. Will fix

Fixed in next version