This just literally saved me a ton of time too. Although I also had another issue with wither skeletons...
[22:16:43] [server thread/INFO]: [J0nis: Removed 19634 entities. (§c1§fx§eSkeleton§f, §c19633§fx§ewitherSkeleton§f)]
Put false for: replacewitherskeletons and got rid of this:
[22:38:35] [server thread/ERROR]: "Silently" catching entity tracking error.
net.minecraft.util.ReportedException: Adding entity to track
at net.minecraft.entity.EntityTracker.func_72785_a(EntityTracker.java:230) [mn.class:?]
at cpw.mods.fml.common.registry.EntityRegistry.tryTrackingEntity(EntityRegistry.java:364) [EntityRegistry.class:?]
at net.minecraft.entity.EntityTracker.func_72786_a(EntityTracker.java:61) [mn.class:?]
at net.minecraft.world.WorldManager.func_72703_a(SourceFile:31) [mp.class:?]
at net.minecraft.world.World.func_72923_a(World.java:1347) [ahb.class:?]
at net.minecraft.world.WorldServer.func_72923_a(WorldServer.java:844) [mt.class:?]
at net.minecraft.world.World.func_72868_a(World.java:3304) [ahb.class:?]
at net.minecraft.world.chunk.Chunk.func_76631_c(Chunk.java:921) [apx.class:?]
at net.minecraftforge.common.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:46) [ChunkIOProvider.class:?]
at net.minecraftforge.common.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:12) [ChunkIOProvider.class:?]
at net.minecraftforge.common.util.AsynchronousExecutor.skipQueue(AsynchronousExecutor.java:344) [AsynchronousExecutor.class:?]
at net.minecraftforge.common.util.AsynchronousExecutor.getSkipQueue(AsynchronousExecutor.java:302) [AsynchronousExecutor.class:?]
at net.minecraftforge.common.chunkio.ChunkIOExecutor.syncChunkLoad(ChunkIOExecutor.java:12) [ChunkIOExecutor.class:?]
at net.minecraft.world.gen.ChunkProviderServer.loadChunk(ChunkProviderServer.java:126) [ms.class:?]
at net.minecraft.world.gen.ChunkProviderServer.func_73158_c(ChunkProviderServer.java:101) [ms.class:?]
at net.minecraft.world.gen.ChunkProviderServer.func_73154_d(ChunkProviderServer.java:199) [ms.class:?]
at net.minecraft.world.World.func_72964_e(World.java:419) [ahb.class:?]
at net.minecraft.world.World.func_147439_a(World.java:345) [ahb.class:?]
at net.minecraft.block.BlockMushroom.func_149674_a(BlockMushroom.java:36) [amc.class:?]
at net.minecraft.world.WorldServer.func_147456_g(WorldServer.java:391) [mt.class:?]
at net.minecraft.world.WorldServer.func_72835_b(WorldServer.java:183) [mt.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:625) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:334) [lt.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:547) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685) [li.class:?]
Caused by: java.lang.IllegalStateException: Entity is already tracked!
at net.minecraft.entity.EntityTracker.func_72785_a(EntityTracker.java:196) ~[mn.class:?]
... 25 more
I love forums.. hah