When you are using ProGuard for obfuscation of some of your NetBeans module code on a Mac running OS X and you will most likely refer to rt.jar in your build.xml and get and error about missing rt.jar.
One quick fix solving this could be the following which helped me.
On your Mac open a terminal and change directory to
now just softlink to classes.jar via
me$ sudo ln -s ../../Classes/classes.jar rt.jar