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I've been making a modpack for a few friends when I came across this this.

2013-10-21 19:39:15 [SEVERE] [ForgeModLoader] Unable to launch
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:131)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
Caused by: java.lang.NoClassDefFoundError: cpw/mods/fml/common/registry/GameData
at net.minecraft.item.Item.<init>(Item.java:274)
at net.minecraft.item.ItemTool.<init>(ItemTool.java:28)
at net.minecraft.item.ItemSpade.<init>(SourceFile:11)
at net.minecraft.item.Item.<clinit>(Item.java:48)
at net.minecraft.block.Block.<clinit>(Block.java:1426)
at net.minecraft.stats.StatList.func_75921_a(StatList.java:185)
at net.minecraft.stats.StatList.<clinit>(StatList.java:96)
at net.minecraft.client.Minecraft.<init>(Minecraft.java:329)
at net.minecraft.client.main.Main.main(SourceFile:83)
... 6 more
Caused by: java.lang.ClassNotFoundException: cpw.mods.fml.common.registry.GameData
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:186)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 15 more
Caused by: java.lang.NullPointerException
at codechicken.core.asm.FeatureHackTransformer.transformer001(FeatureHackTransformer.java:31)
at codechicken.core.asm.FeatureHackTransformer.transform(FeatureHackTransformer.java:42)
at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:274)
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:172)
... 17 more

If some one would like to tell me what the error means that would be greatly appreciated

Looks like one of the mods uses an old version.

Thank you