Annoying Problem

[Tootsie Roll]

On my webclient there is this error.. which is this.
java.lang.ClassFormatError: Incompatible magic value 1008813135 in class file client
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknow n Source)
at sun.plugin2.applet.Applet2ClassLoader.findClass(Un known Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.plugin2.applet.Plugin2ClassLoader.loadCode(Unk nown Source)
at sun.plugin2.applet.Plugin2Manager.createApplet(Unk nown Source)
at sun.plugin2.applet.Plugin2Manager$AppletExecutionR unnable.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Exception: java.lang.ClassFormatError: Incompatible magic value 1008813135 in class file client

I know what the problem is.. the problem is Incompatible magic value 1008813135 but I have no idea how to fix it.. I've tried several times and it still hasn't work.