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Client> Jan 02, 2015 7:31:46 PM net.minecraft.launchwrapper.LogWrapper log
Client> INFO: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Client> Jan 02, 2015 7:31:46 PM net.minecraft.launchwrapper.LogWrapper log
Client> INFO: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Client> Jan 02, 2015 7:31:46 PM net.minecraft.launchwrapper.LogWrapper log
Client> INFO: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker
Client> 2015-01-02 19:31:47 [INFO] [ForgeModLoader] Forge Mod Loader version 6.4.49.965 for Minecraft 1.6.4 loading
Client> 2015-01-02 19:31:47 [INFO] [ForgeModLoader] Java is Java HotSpot(TM) 64-Bit Server VM, version 1.8.0_25, running on Mac OS X:x86_64:10.9.5, installed at /Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home
Client> 2015-01-02 19:31:47 [INFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
Client> 2015-01-02 19:31:47 [INFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker
Client> 2015-01-02 19:31:47 [INFO] [ForgeModLoader] Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
Client> 2015-01-02 19:31:47 [SEVERE] [ForgeModLoader] Unable to launch
Client> java.util.ConcurrentModificationException
Client> at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
Client> at java.util.ArrayList$Itr.remove(ArrayList.java:865)
Client> at net.minecraft.launchwrapper.Launch.launch(Launch.java:114)
Client> at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
Game ended with no troubles detected (exit code 0)
Deleting /Users/Khirby/Library/Application Support/minecraft/versions/1.6.4-Forge9.11.1.965/1.6.4-Forge9.11.1.965-natives-15104622431889

java 8 isnt compatible with forge yet you need to change to java 7