java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:926)
	at java.util.HashMap$KeyIterator.next(HashMap.java:960)
	at net.minecraft.server.v1_7_R1.World.tickEntities(World.java:1353)
	at net.minecraft.server.v1_7_R1.WorldServer.tickEntities(WorldServer.java:514)
	at net.minecraft.server.v1_7_R1.MinecraftServer.u(MinecraftServer.java:671)
	at net.minecraft.server.v1_7_R1.DedicatedServer.u(DedicatedServer.java:273)
	at net.minecraft.server.v1_7_R1.MinecraftServer.t(MinecraftServer.java:560)
	at net.minecraft.server.v1_7_R1.MinecraftServer.run(MinecraftServer.java:467)
	at net.minecraft.server.v1_7_R1.ThreadServerApplication.run(SourceFile:617)