The default is presently 1.7, even if your java is 1.6, which gives you magic number errors when dev mode (test or debug) generates the JavaScriptObject$ type. My intent had been to paper over this backward incompatible change - I'll be submitting a patch for the arg handler itself in the morning, see https://groups.google.com/d/topic/google-web-toolkit-contributors/nRKTvV2iBUs/discussion for more discussion.