My test world seems to have gone bust:
2013-10-31 15:34:59 [INFO] [STDERR] t: Exception ticking world
2013-10-31 15:34:59 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.r(MinecraftServer.java:675)
2013-10-31 15:34:59 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:599)
2013-10-31 15:34:59 [INFO] [STDERR] at bdz.q(IntegratedServer.java:170)
2013-10-31 15:34:59 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:497)
2013-10-31 15:34:59 [INFO] [STDERR] at fy.run(SourceFile:849)
2013-10-31 15:34:59 [INFO] [STDERR] Caused by: java.lang.RuntimeException: java.lang.NullPointerException
2013-10-31 15:34:59 [INFO] [STDERR] at ic2.core.IC2.onLivingSpecialSpawn(IC2.java:2270)
2013-10-31 15:34:59 [INFO] [STDERR] at net.minecraftforge.event.ASMEventHandler_32_IC2_onLivingSpecialSpawn_LivingSpecialSpawnEvent.invoke(.dynamic)
2013-10-31 15:34:59 [INFO] [STDERR] at net.minecraftforge.event.ASMEventHandler.invoke(ASMEventHandler.java:35)
2013-10-31 15:34:59 [INFO] [STDERR] at net.minecraftforge.event.EventBus.post(EventBus.java:103)
2013-10-31 15:34:59 [INFO] [STDERR] at net.minecraftforge.event.ForgeEventFactory.doSpecialSpawn(ForgeEventFactory.java:52)
2013-10-31 15:34:59 [INFO] [STDERR] at yp.a(SpawnerAnimals.java:245)
2013-10-31 15:34:59 [INFO] [STDERR] at yp.a(SpawnerAnimals.java:187)
2013-10-31 15:34:59 [INFO] [STDERR] at in.b(WorldServer.java:162)
2013-10-31 15:34:59 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.r(MinecraftServer.java:669)
2013-10-31 15:34:59 [INFO] [STDERR] ... 4 more
2013-10-31 15:34:59 [INFO] [STDERR] Caused by: java.lang.NullPointerException
2013-10-31 15:34:59 [INFO] [STDERR] at ic2.core.IC2.onLivingSpecialSpawn(IC2.java:2268)
2013-10-31 15:34:59 [INFO] [STDERR] ... 12 more
Strange it just started happening.