When I select the forge profile (using 1.6.4 as I'm trying to get crazycraft to work) and press play minecraft closes for no apparent reason and no error messages appear. Minecraft opens normally with my account and I am using the latest version of forge for 1.6.4. If I keep the launcher open this message appears under the log:
Sep 13, 2014 6:30:09 PM net.minecraft.launchwrapper.LogWrapper log
INFO: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Sep 13, 2014 6:30:09 PM net.minecraft.launchwrapper.LogWrapper log
INFO: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Sep 13, 2014 6:30:09 PM net.minecraft.launchwrapper.LogWrapper log
INFO: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker
2014-09-13 18:30:10 [iNFO] [ForgeModLoader] Forge Mod Loader version 6.4.49.965 for Minecraft 1.6.4 loading
2014-09-13 18:30:10 [iNFO] [ForgeModLoader] Java is Java HotSpot 64-Bit Server VM, version 1.8.0_20, running on Windows 8.1:amd64:6.3, installed at C:\Program Files\Java\jre1.8.0_20
2014-09-13 18:30:10 [iNFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
2014-09-13 18:30:10 [iNFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker
2014-09-13 18:30:10 [iNFO] [ForgeModLoader] Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
2014-09-13 18:30:10 [sEVERE] [ForgeModLoader] Unable to launch
java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.remove(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:114)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
Java HotSpot 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
I have tried using both java runtime environment 7 and 8 and am not sure what to do