NetBeans 6.5 Beta quick tip
By simonri on Oct 17, 2008
Googling the error message turned up a lot of results related to some stubs jar files in Maven which clearly was not related to my problem.
Further searching led me to a forum entry in German, the crux of which was that this was a bug in the compile-on-save functionality. To fix this you simply need to right-click the project you're trying to run and select properties. In the Compiling section under Build there is a check-box for "Compile on Save". Deselect this and voila! the application runs without problem.