Some people on my server have been having this issue for the past while now(and past few versions). Hoping there's a fix (or will be)
---- Minecraft Crash Report ----
// Uh... Did I do that?
Time: 2/8/15 1:58 PM
Description: Unexpected error
java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at fastcraft.a.m.a(F:51)
at fastcraft.Hooks.o(F:124)
at net.minecraft.world.World.func_72939_s(World.java:1989)
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1994)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:961)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:887)
at net.minecraft.client.main.Main.main(SourceFile:148)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:286)
at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:376)
at org.multimc.EntryPoint.listen(EntryPoint.java:165)
at org.multimc.EntryPoint.main(EntryPoint.java:54)
Any thoughts?