This stack trace may be from an old version.

2011-06-28 14:15:03 [SEVERE] Could not pass event CREATURE_SPAWN to WorldGuard java.lang.NullPointerException at com.sk89q.worldguard.bukkit.WorldGuardEntityListener.onCreatureSpawn(WorldGuardEntityListener.java:552) at org.bukkit.plugin.java.JavaPluginLoader$63.execute(JavaPluginLoader.java:684) at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:58) at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:321) at org.bukkit.craftbukkit.event.CraftEventFactory.callCreatureSpawnEvent(CraftEventFactory.java:234) at net.minecraft.server.World.addEntity(World.java:846) at net.minecraft.server.SpawnerCreature.spawnEntities(SpawnerCreature.java:134) at net.minecraft.server.World.doTick(World.java:1623) at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:441) at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:361) at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)

Kane_Hart said he got this today. I've also gotten it, but no more often that once per world load

I've tested, seems like this is fixed.