EDIT: This has been fixed in the newer versions such as 1.5.1 and works as expected. Thanks Mojang.

I have been working on the animations created with spawners.
The spawners spawn very short living "sand" entities.

This should result in a stable entity count.
But as soon as you move away from the place where the entities get spawned they stop despawning + spawners still spawn and therefore creating massive amounts of entites which results in lag or even game corruption.

What I expected to happen was:
Despawing of entities with a short live time at gaining range.

What actually happened:
Entities with short "live" won't despawn at a certain range. Probably if they are in the farthest loaded chunk.
Edit: Testing gave me reason to believe it might also have to do with spawners spawning entities without being loaded. E.g.: Same problem appears with render distance on tiny mixed with distance of about 250 blocks.

Steps to reproduce:
This video shows the bug and how it can be created.: http://youtu.be/15fb5YiEJQo
Interesting part at: 10:28 (bug demonstration)

Feel free to tweet @Sralam_ if you need more information.