I'm using Minecraft forge-1.6.4-9.11.1.965-src and when I try to debug on eclipse this pops up in the console:
Apr 11, 2014 8:00:33 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Apr 11, 2014 8:00:33 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Apr 11, 2014 8:00:33 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker
Java HotSpot Client VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
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