When too many mobs are put in a confined area, and that chunk is loaded (the player logs in, or enters the area otherwise), the server suddenly starts using all the memory available on the server, and eventually goes to swap, then java closes minecraft with a memory exception. Specifically, one of our players had more than 400 cows in a 1x1 box. Another player had hundreds of sheep in 2x2 fenced yards. It seems to be more of a problem with cows and sheep than chickens. As soon as these redstone "factory farms" were removed, the chunks can load, and server is back at regular RAM usage (around 3GB).

What I Expected to Happen:
I expect Minecraft server to use a reasonable amount of RAM, and that the RAM used would not suddenly increase to the point where the server crashed.

What Happened:
One of our players put 400+ cows in a little box and they were glitching so hard trying to find a path that their AI code grabbed all the RAM on the box and forced java to end the process.

How to Reproduce:
Stuff about 400 cows in a 1x1 box (they must be bred and be forced in by water)..... OR...... breed about 500 sheep in a 2x2 fenced area.