I've got this exception when I added a new module
to the system, which had the same code name as
some old one and removed the old one.

Steps to reproduce:
Run build #200312101900 with an empty user dir.
Mount one or two JavaCVS filesystems and exit.

Remove modules/javacvs.jar and
modules/ext/cvsclient.jar from the installation
directory. Add javacvscompat.jar into modules/
folder. You get javacvscompat.jar by building
javacvs/compat module.

Start the IDE with the same user dir.

During the IDE start following messages were
printed into ide.log:
Warning: the module
/home/me97925/NetBeansIDE-dev-200312101900/netbeans/modules/javacvs.jar
could not be found, ignoring...
New modules:
        org.netbeans.modules.cvsclient/1 [2.0
${buildnumber}]

This is O.K.

But after modules were turned on:
Turning on modules:
        org.openide/1 [4.17 200312101900]
        ...
        org.netbeans.modules.web.taglibed/1 [1.12
200312101900]
Turning on modules:
        org.netbeans.modules.cvsclient/1 [2.0
${buildnumber}]

the ClassCastException was thrown and IDE did not
start. The main() was interrupted.

I had to kill the IDE. The second start was
successfull.