error

[15:55:43] [Server thread/FATAL] [FML]: Exception caught executing FutureTask: java.util.concurrent.ExecutionException: java.util.ConcurrentModificationException
java.util.concurrent.ExecutionException: java.util.ConcurrentModificationException
	at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_45]
	at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_45]
	at net.minecraftforge.fml.common.FMLCommonHandler.callFuture(FMLCommonHandler.java:715) [FMLCommonHandler.class:?]
	at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:727) [MinecraftServer.class:?]
	at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:669) [MinecraftServer.class:?]
	at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:171) [IntegratedServer.class:?]
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:540) [MinecraftServer.class:?]
	at java.lang.Thread.run(Thread.java:745) [?:1.8.0_45]
Caused by: java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901) ~[?:1.8.0_45]
	at java.util.ArrayList$Itr.next(ArrayList.java:851) ~[?:1.8.0_45]
	at com.smashmod.items.MetaSword.onItemRightClick(MetaSword.java:58) ~[MetaSword.class:?]
	at net.minecraft.item.ItemStack.useItemRightClick(ItemStack.java:168) ~[ItemStack.class:?]
	at net.minecraft.server.management.ItemInWorldManager.tryUseItem(ItemInWorldManager.java:378) ~[ItemInWorldManager.class:?]
	at net.minecraft.network.NetHandlerPlayServer.processPlayerBlockPlacement(NetHandlerPlayServer.java:607) ~[NetHandlerPlayServer.class:?]
	at net.minecraft.network.play.client.C08PacketPlayerBlockPlacement.processPacket(C08PacketPlayerBlockPlacement.java:67) ~[C08PacketPlayerBlockPlacement.class:?]
	at net.minecraft.network.play.client.C08PacketPlayerBlockPlacement.processPacket(C08PacketPlayerBlockPlacement.java:114) ~[C08PacketPlayerBlockPlacement.class:?]
	at net.minecraft.network.PacketThreadUtil$1.run(PacketThreadUtil.java:24) ~[PacketThreadUtil$1.class:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_45]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_45]
	at net.minecraftforge.fml.common.FMLCommonHandler.callFuture(FMLCommonHandler.java:714) ~[FMLCommonHandler.class:?]
	... 5 more