I'm using the latest version of Forge, MC 1.4.4, and Magic Launcher.
Its a Forge error because I removed Forge from the internal mods list, and it worked fine, well except my mods.
*** MagicMinecraftLauncher 1.0.0 ***
java.lang.IncompatibleClassChangeError: Implementing class
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at magic.launcher.Launcher.main(Unknown Source)
I also have a clean Minecraft jar, and have tried redownloading Magic Launcher (on different versions also).