Cow Pond: Funny Glitch

There was an error on my CraftBukkit server with cow spawning (something about Entity Cow not in correct chunk), so I went to the coordinates to check it out. This is what I found:

A few hundred cows on the exact same spot. The game started lagging hard at this point. 

I climbed up to look at them from above and saw a "Cow Monster".

So, naturally, I decided to start killing them...

After killing a few dozen, all the cows instantly spread out in the vicinity!

... filling up the pond and surrounding area ...

... bobbing up and down in the water, giving me the idea to call it "Cow Pond".

The view from another angle.

I had a couple of these happen in different places on the server. Has anyone else seen this? Minecraft version 1.5.2. CraftBukkit version 1.5.2-R0.1.