I too am using magic launcher 1.0.0 for minecraft 1.4.2 I loaded up an earlier version of forge and everything seemed to work except for ID resolver and Auto Switch. After reading some posts there was information that the auto switch bug was fixed in the latest. So I downloaded, started with a fresh brand new .jar file. I loaded up Magic Launcher and Forge Universal 350 (or any before it) and got the error:
*** 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)
Any ideas on how to fix? Thanks!