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I'm trying to regress to 1.6.4 but am crashing trying to launch. I've emptied out the mods folder but still cannot launch. Any clues form this log?

Jan 30, 2015 11:03:50 PM net.minecraft.launchwrapper.LogWrapper log
INFO: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Jan 30, 2015 11:03:50 PM net.minecraft.launchwrapper.LogWrapper log
INFO: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Jan 30, 2015 11:03:50 PM net.minecraft.launchwrapper.LogWrapper log
INFO: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker
2015-01-30 23:03:50 [INFO] [ForgeModLoader] Forge Mod Loader version 6.4.49.965 for Minecraft 1.6.4 loading
2015-01-30 23:03:50 [INFO] [ForgeModLoader] Java is Java HotSpot(TM) Client VM, version 1.8.0_25, running on Windows 7:x86:6.1, installed at C:\Program Files (x86)\Java\jre1.8.0_25
2015-01-30 23:03:50 [INFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
2015-01-30 23:03:50 [INFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker
2015-01-30 23:03:50 [INFO] [ForgeModLoader] Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
2015-01-30 23:03:50 [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(TM) Client VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

Quote from grimallq»

Legacy Java Fixer.

How would I fix this?

Got it. Thanks a bunch.