Today I downloaded the recommended forge build (universal) for minecraft 1.5.1. I tried to launch it with magic launcher without any other mods and a clean minecraft.jar (1.5.1). Minecraft didn't launch and I received this log:

 

*** 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)

 

 

 

 

It only happens with forge.

You've installed the wrong version of forge for the wrong version of minecraft, yes you say you're doing 1.5.1 vs 1.5.1 but your error log states otherwise.

I'm 100% sure I have minecraft 1.5.1 because I force updated minecraft and in the main menu it says 1.5.1.

And for forge I have the "minecraftforge-universal-1.5.1-7.7.1.662.zip"

Then speak with the Magiclauncher people.. it would be an issue on there end. If you're absolutely sure its not your issue.

I'm fairly sure its your issue as many people use MagicLauncher with Forge just fine...

 

Either way it's not our issue. The install process is relay easy, just use our classes over the vanilla classes.