We had three server lockups today:

15:42:06 [WARNING] Wrong location for net.minecraft.server.EntityCow@baf9 in world 'world_suk'!
15:42:06 [WARNING] Entity is at -Infinity,Infinity (chunk 2147483647,2147483647) but was stored in chunk 2,-1

Cause came out later:

[16:05] Basically it must be something to do with mob riding. I was on my world, I got on the cow using the saddle, and then was riding around for a bit. It then died and i was still sitting down. about 20 seconds later the server crashed

I'm having the same issue. I think that mob is still sitting somewhere in the world and needs to be removed.

19:59:47 [WARNING] Wrong location for net.minecraft.server.EntityWolf@2c3dc in world 'world'!
19:59:47 [WARNING] Entity is at -Infinity,-Infinity (chunk 2147483647,2147483647) but was stored in chunk 590,18

I think what happened is the mob got pushed under the map.

http://forums.bukkit.org/threads/infinity-infinity-help.83742/