After trying a modpack, ALL the different versions of Forge stopped working for me, and for all the modpacks, I got the same error. Here's one of them:

 

 

 

[2014-04-23 15:39:51] Logging into Minecraft!

[2014-04-23 15:39:52] Launching Minecraft with the following arguments (user related stuff has been removed): [C:\Program Files\Java\jre1.8.0_20\bin\javaw, -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump, -Xms256M, -Xmx1024M, -XX:PermSize=512M, -Dfml.ignorePatchDiscrepancies=true, -Dfml.ignoreInvalidMinecraftCertificates=true, -Dfml.log.level=INFO, -Djava.library.path=C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\natives, -cp, C:\Users\Matthew\Downloads\ATLauncher\ATLauncher.exe;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\jarmods\forge-1.6.4-9.11.1.965-universal.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\launchwrapper-1.8.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\asm-all-4.1.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\scala-library-2.10.2.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\scala-compiler-2.10.2.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\lzma-0.0.1.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\jopt-simple-4.5.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\jopt-simple-4.5.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\codecjorbis-20101023.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\codecwav-20101023.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\libraryjavasound-20101123.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\librarylwjglopenal-20100824.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\soundsystem-20120107.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\argo-2.25_fixed.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\bcprov-jdk15on-1.47.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\guava-14.0.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\commons-lang3-3.1.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\commons-io-2.4.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\jinput-2.0.5.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\jutils-1.0.0.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\gson-2.2.2.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\lwjgl-2.9.0.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\lwjgl_util-2.9.0.jar;C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness\bin\minecraft.jar, net.minecraft.launchwrapper.Launch, --username, REDACTED, --session, token:REDACTED:REDACTED, --version, 1.6.4, --gameDir, C:\Users\Matthew\Downloads\ATLauncher\Instances\FromTheDarkness, --assetsDir, C:\Users\Matthew\Downloads\ATLauncher\Configs\Resources\virtual\legacy, --width=854, --height=480, --tweakClass=cpw.mods.fml.common.launcher.FMLTweaker]

[2014-04-23 15:39:56] Apr 23, 2014 3:39:55 PM net.minecraft.launchwrapper.LogWrapper log

[2014-04-23 15:39:56] INFO: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker

[2014-04-23 15:39:56] Apr 23, 2014 3:39:56 PM net.minecraft.launchwrapper.LogWrapper log

[2014-04-23 15:39:56] INFO: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker

[2014-04-23 15:39:56] Apr 23, 2014 3:39:56 PM net.minecraft.launchwrapper.LogWrapper log

[2014-04-23 15:39:56] INFO: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker

[2014-04-23 15:39:57] [iNFO] [ForgeModLoader] Forge Mod Loader version 6.4.49.965 for Minecraft 1.6.4 loading

[2014-04-23 15:39:57] [iNFO] [ForgeModLoader] Java is Java HotSpot Client VM, version 1.8.0_20-ea, running on Windows 7:x86:6.1, installed at C:\Program Files\Java\jre1.8.0_20

[2014-04-23 15:39:57] [WARNING] [ForgeModLoader] The coremod codechicken.core.launch.DepLoader does not have a MCVersion annotation, it may cause issues with this version of Minecraft

[2014-04-23 15:39:58] [WARNING] [ForgeModLoader] The coremod appeng.transformer.AppEngCore does not have a MCVersion annotation, it may cause issues with this version of Minecraft

[2014-04-23 15:39:58] [iNFO] [ForgeModLoader] [AppEng] Core Init

[2014-04-23 15:39:58] [WARNING] [ForgeModLoader] The coremod codechicken.core.launch.CodeChickenCorePlugin does not have a MCVersion annotation, it may cause issues with this version of Minecraft

[2014-04-23 15:40:03] [iNFO] [sTDERR] Warning: version of CodeChickenLib-universal-1.6.4-, 1.0.0.49 is newer than request 1.0.0.43

[2014-04-23 15:40:03] [WARNING] [ForgeModLoader] The coremod invtweaks.forge.asm.FMLPlugin does not have a MCVersion annotation, it may cause issues with this version of Minecraft

[2014-04-23 15:40:03] [WARNING] [ForgeModLoader] The coremod mcp.mobius.mobiuscore.asm.CoreDescription does not have a MCVersion annotation, it may cause issues with this version of Minecraft

[2014-04-23 15:40:03] [WARNING] [ForgeModLoader] The coremod codechicken.nei.asm.NEICorePlugin does not have a MCVersion annotation, it may cause issues with this version of Minecraft

[2014-04-23 15:40:03] [WARNING] [ForgeModLoader] The coremod tconstruct.preloader.TConstructLoaderContainer does not have a MCVersion annotation, it may cause issues with this version of Minecraft

[2014-04-23 15:40:04] [iNFO] [TCon-Preloader] Scalpel. Suction. Lumber axe. CLEAR! *zap*

[2014-04-23 15:40:04] [iNFO] [TCon-Preloader] Constructing preloader (Modules: [tconstruct.preloader.AccessTransformers, tconstruct.preloader.ASMInterfaceRepair])

[2014-04-23 15:40:04] [iNFO] [TCon-Preloader] Found a properties file. Attempting load...

[2014-04-23 15:40:04] [iNFO] [TCon-Preloader] Loaded properties successfully. Using specified settings.

[2014-04-23 15:40:04] [WARNING] [ForgeModLoader] The coremod com.chunkbase.mod.forge.mods.unglitch.UnglitchLoader does not have a MCVersion annotation, it may cause issues with this version of Minecraft

[2014-04-23 15:40:04] [iNFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker

[2014-04-23 15:40:04] [iNFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker

[2014-04-23 15:40:04] [iNFO] [ForgeModLoader] Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker

[2014-04-23 15:40:04] [sEVERE] [ForgeModLoader] Unable to launch

[2014-04-23 15:40:04] java.util.ConcurrentModificationException

[2014-04-23 15:40:04] at java.util.ArrayList$Itr.checkForComodification(Unknown Source)

[2014-04-23 15:40:04] at java.util.ArrayList$Itr.remove(Unknown Source)

[2014-04-23 15:40:04] at net.minecraft.launchwrapper.Launch.launch(Launch.java:114)

[2014-04-23 15:40:04] at net.minecraft.launchwrapper.Launch.main(Launch.java:27)

[2014-04-23 15:40:04] Java HotSpot Client VM warning: ignoring option PermSize=512M; support was removed in 8.0

 

 

 

The same "[sEVERE] [ForgeModLoader] Unable to launch - java.util.ConcurrentModificationException" keeps happening now, ever since I used the From the Darkness modpack, and I still haven't been able to fix it D:

 

Someone please help, and thanks in advance

Don't use Java 8

Before I started using Java 8, when I used Java 7, I always got this error message:

 

Error: Could not create the Java Virtual Machine

Error: A fatal exception has occurred. Program will exit

 

And I could never fix that, even with the Xmx or Xms Java commands. The only other thing I could think to do is to find a way with Java 8, or use Java 6

It turns out I was right. After 2 and a half hours of screwing around with oracle, trying to get Java 6, I finally got it. Java 6 worked, but not the first time, which is fine since it's not crashing the way it did the very first time. Thanks for the help.