"main" java.lang.AbstractMethodError

[Former user]

Every time I try to play from the launcher this comes up:

Exception in thread "main" java.lang.AbstractMethodError
    at cpw.mods.fml.relauncher.CoreModManager.loadCoreMod(CoreModManager.java:423)
    at cpw.mods.fml.relauncher.CoreModManager.discoverCoreMods(CoreModManager.java:310)
    at cpw.mods.fml.relauncher.CoreModManager.handleLaunch(CoreModManager.java:211)
    at cpw.mods.fml.relauncher.FMLLaunchHandler.setupHome(FMLLaunchHandler.java:90)
    at cpw.mods.fml.relauncher.FMLLaunchHandler.setupClient(FMLLaunchHandler.java:67)
    at cpw.mods.fml.relauncher.FMLLaunchHandler.configureForClientLaunch(FMLLaunchHandler.java:34)
    at cpw.mods.fml.common.launcher.FMLTweaker.injectIntoClassLoader(FMLTweaker.java:126)
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:114)
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

PLEASE HELP

[Chimaine]

Please provide the logfiles (on pastebin or similar) and the version of Forge/MobTalker2 you're using.