6.0 M2 & M3 occassionally throw the following exception when expanding folders
under Files:

java.lang.ClassNotFoundException:
org.netbeans.modules.java.imptool.ImpToolSettings from
SystemClassLoader[org.openide.util,org.openide.modules,org.openide.awt,org.openide.options,org.netbeans.api.progress,org.openide.dialogs,org.openide.nodes,org.openide.explorer,org.openide.windows,org.netbeans.modules.editor.mimelookup,org.openide.text,org.openide.actions,org.openide.filesystems,org.openide.io,org.openide.execution,org.netbeans.api.java,org.openide.loaders,org.openide.src,org.netbeans.modules.queries,org.netbeans.bootstrap,org.netbeans.core.startup,org.netbeans.swing.plaf,org.netbeans.core,org.netbeans.modules.javahelp,org.openidex.util,javax.jmi.reflect,org.netbeans.api.mdr,org.netbeans.jmi.javamodel,org.netbeans.modules.classfile,javax.jmi.model,org.netbeans.modules.jmiutils,org.netbeans.modules.mdr,org.netbeans.modules.projectapi,org.netbeans.modules.javacore,org.netbeans.modules.editor.errorstripe.api,org.netbeans.modules.java,org.netbeans.modules.projectuiapi,org.netbeans.modules.java.platform,org.netbeans.modules.project.ant,org.netbeans.modules.options.api,org.jdesktop.layout,org.netbeans.api.xml,org.netbeans.spi.navigator,org.apache.tools.ant.module,org.netbeans.modules.project.libraries,org.netbeans.modules.java.project,org.netbeans.modules.properties,org.netbeans.modules.i18n,org.netbeans.modules.editor.util,org.netbeans.modules.editor.fold,org.netbeans.modules.editor.lib,org.netbeans.spi.palette,org.netbeans.modules.editor.settings,org.netbeans.modules.editor,org.netbeans.modules.editor.completion,org.netbeans.modules.html.editor.lib,org.netbeans.modules.html,org.netbeans.modules.editor.errorstripe,org.netbeans.modules.xml.core,org.netbeans.libs.xerces,org.netbeans.modules.xml.tax,org.netbeans.modules.apisupport.ant,org.netbeans.core.multiview,org.netbeans.modules.junit,org.netbeans.api.debugger,org.netbeans.spi.viewmodel,org.netbeans.api.debugger.jpda,org.netbeans.modules.debugger.jpda.ant,org.netbeans.modules.ant.browsetask,org.netbeans.modules.apisupport.project,org.netbeans.modules.diff,org.netbeans.libs.svnClientAdapter,org.netbeans.modules.versioning,org.netbeans.modules.masterfs,org.netbeans.libs.ini4j,org.netbeans.modules.subversion,org.netbeans.modules.javadoc,org.netbeans.modules.schema2beans,org.netbeans.api.web.webmodule,org.netbeans.modules.java.editor.lib,org.netbeans.modules.xml.catalog,org.netbeans.modules.editor.codetemplates,org.netbeans.modules.java.editor,org.netbeans.modules.j2ee.dd,org.netbeans.modules.j2ee.common,org.netbeans.modules.web.jsf,org.netbeans.modules.servletapi,org.netbeans.modules.httpserver,org.netbeans.modules.web.monitor,org.netbeans.modules.ant.freeform,org.netbeans.modules.java.freeform,org.netbeans.modules.web.freeform,org.netbeans.modules.websvc.clientapi,org.netbeans.modules.java.j2seproject,org.netbeans.modules.xml.multiview,org.netbeans.modules.xml.schema,org.netbeans.modules.java.j2seplatform,org.netbeans.spi.debugger.ui,org.netbeans.modules.debugger.jpda.projects,org.netbeans.modules.debugger.jpda,org.netbeans.modules.debugger.jpda.ui,org.netbeans.modules.utilities,org.netbeans.core.ui,org.netbeans.modules.db,org.netbeans.modules.derby,org.netbeans.modules.favorites,org.netbeans.core.execution,org.netbeans.modules.projectui,org.netbeans.modules.j2ee.dd.webservice,org.netbeans.modules.websvc.websvcapi,org.netbeans.modules.j2ee.sun.dd,org.netbeans.modules.dbschema,org.netbeans.modules.settings,org.netbeans.modules.j2eeapis,org.netbeans.modules.j2eeserver,org.netbeans.modules.jmxri,org.netbeans.modules.j2ee.sun.appsrv,org.netbeans.modules.j2ee.sun.ddui,org.netbeans.modules.editor.structure,org.netbeans.modules.j2ee.ant,org.netbeans.tasklistapi,org.netbeans.modules.tasklist.core,org.netbeans.modules.suggestions_framework,org.netbeans.modules.editor.hints,org.netbeans.modules.j2ee.blueprints,org.netbeans.modules.editor.mimelookup.impl,org.netbeans.modules.java.hints,org.netbeans.modules.servletapi24,org.netbeans.libs.commons_logging,org.netbeans.modules.web.jspparser,org.netbeans.lib.cvsclient,org.netbeans.libs.jsch,org.netbeans.modules.versioning.system.cvss,org.netbeans.modules.editor.plain.lib,org.netbeans.modules.xml.text,org.netbeans.modules.html.editor,org.netbeans.modules.web.core.syntax,org.netbeans.modules.web.struts,org.netbeans.modules.web.debug,org.netbeans.modules.editor.settings.storage,org.netbeans.modules.autoupdate,org.netbeans.modules.options.editor,org.netbeans.modules.j2ee.sun.appsrv81,org.netbeans.modules.refactoring,org.netbeans.modules.apisupport.refactoring,org.netbeans.modules.ant.grammar,org.netbeans.upgrader,org.netbeans.modules.j2ee.api.ejbmodule,org.netbeans.modules.web.core,org.netbeans.modules.websvc.registry,org.netbeans.modules.websvc.core,org.netbeans.modules.j2ee.refactoring,org.netbeans.modules.usersguide,org.netbeans.modules.beans,org.netbeans.modules.utilities.project,org.netbeans.modules.j2ee.ejbcore,org.netbeans.modules.j2ee.ddloaders,org.netbeans.modules.progress.ui,org.netbeans.modules.web.project,org.netbeans.modules.j2ee.ejbjarproject,org.netbeans.modules.j2ee.earproject,org.netbeans.core.output2,org.netbeans.modules.properties.syntax,org.netbeans.modules.apisupport.feedreader,org.netbeans.modules.j2ee.ejbfreeform,org.netbeans.modules.db.core,org.netbeans.modules.tomcat5,org.netbeans.modules.tomcat5.bundled,org.netbeans.modules.j2ee.jboss4,org.netbeans.modules.image,org.netbeans.modules.xml.tools,org.netbeans.modules.j2ee.weblogic9,org.netbeans.modules.editor.bookmarks,org.netbeans.modules.web.examples,org.netbeans.modules.clazz,org.netbeans.modules.tasklist.docscan,org.netbeans.modules.form,org.netbeans.modules.updatecenters,org.netbeans.modules.j2ee.debug,org.netbeans.modules.editor.plain,org.netbeans.modules.web.jstl11,org.netbeans.modules.defaults,org.netbeans.core.ide,org.netbeans.modules.websvc.dev,org.netbeans.modules.extbrowser,org.netbeans.modules.java.examples,org.netbeans.modules.db.sql.editor,org.netbeans.swing.tabcontrol,org.netbeans.modules.java.navigation,org.netbeans.modules.welcome,org.netbeans.modules.css,org.netbeans.core.windows,org.netbeans.modules.apisupport.paintapp,org.netbeans.modules.xsl,org.netbeans.modules.j2ee.genericserver,org.netbeans.modules.j2ee.platform,org.netbeans.modules.websvc.jaxrpc16,org.netbeans.modules.ant.debugger,org.netbeans.modules.i18n.form]
	at org.netbeans.ProxyClassLoader.loadClass(ProxyClassLoader.java:173)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
	at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:247)
	at
org.openide.util.io.NbObjectInputStream.resolveClass(NbObjectInputStream.java:67)
	at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1575)
	at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1496)
	at java.io.ObjectInputStream.readClass(ObjectInputStream.java:1462)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1312)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1945)
	at java.io.ObjectInputStream.defaultReadObject(ObjectInputStream.java:480)
	at
org.openide.util.SharedClassObject$WriteReplace.readObject(SharedClassObject.java:686)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1846)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1753)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
	at org.openide.util.io.NbMarshalledObject.get(NbMarshalledObject.java:105)
	at
org.openide.options.ContextSystemOption.readExternal(ContextSystemOption.java:164)
	at
org.netbeans.modules.java.settings.JavaSettings.readExternal(JavaSettings.java:216)
	at
org.openide.util.SharedClassObject$WriteReplace.readObject(SharedClassObject.java:703)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1846)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1753)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
	at
org.netbeans.modules.settings.convertors.XMLSettingsSupport$SettingsRecognizer.readSerial(XMLSettingsSupport.java:522)
	at
org.netbeans.modules.settings.convertors.XMLSettingsSupport$SettingsRecognizer.instanceCreate(XMLSettingsSupport.java:554)
	at
org.netbeans.modules.settings.convertors.SerialDataConvertor$SettingsInstance.instanceCreate(SerialDataConvertor.java:382)
	at org.openide.loaders.FolderLookup$ICItem.getInstance(FolderLookup.java:512)
	at org.openide.util.lookup.AbstractLookup$R.allInstances(AbstractLookup.java:897)
	at org.openide.util.lookup.ProxyLookup$R.computeResult(ProxyLookup.java:457)
	at org.openide.util.lookup.ProxyLookup$R.allInstances(ProxyLookup.java:400)
	at org.openide.util.lookup.ProxyLookup$R.computeResult(ProxyLookup.java:457)
	at org.openide.util.lookup.ProxyLookup$R.allInstances(ProxyLookup.java:400)
	at org.openide.util.SharedClassObject.findObject(SharedClassObject.java:535)
	at
org.netbeans.modules.java.settings.JavaSettings.getDefault(JavaSettings.java:235)
	at
org.netbeans.modules.java.JavaNode$StatePropagator.registerBuildStatusListener(JavaNode.java:815)
	at org.netbeans.modules.java.JavaNode$StatePropagator.<init>(JavaNode.java:741)
	at org.netbeans.modules.java.JavaNode$StatePropagator.getDefault(JavaNode.java:756)
	at org.netbeans.modules.java.JavaNode.resolveIcons(JavaNode.java:548)
	at org.netbeans.modules.java.JavaNode$StateUpdater.updateNodes(JavaNode.java:715)
	at org.netbeans.modules.java.JavaNode$StateUpdater.run(JavaNode.java:708)
	at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:541)
[catch] at
org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:963)

Version information from messages.log:

-------------------------------------------------------------------------------
>Log Session: Friday, September 22, 2006 10:58:20 AM CDT
>System Info:
  Product Version         = NetBeans IDE Dev (Build 200609161800)
  Operating System        = Linux version 2.6.12-1.1398_FC4smp running on i386
  Java; VM; Vendor; Home  = 1.6.0-rc; Java HotSpot(TM) Client VM 1.6.0-rc-b96;
Sun Microsystems Inc.; /usr/java/jdk1.6.0/jre
  System Locale; Encoding = en_US (nb); UTF-8
  Home Dir.; Current Dir. = /home/rich; /home/rich
  Installation; User Dir. =
/usr/local/netbeans/NetBeans-IDE-6.0-M3/nb6.0:/usr/local/netbeans/NetBeans-IDE-6.0-M3/ide8:/usr/local/netbeans/NetBeans-IDE-6.0-M3/enterprise4:/usr/local/netbeans/NetBeans-IDE-6.0-M3/harness:/home/rich:/usr/local/netbeans/NetBeans-IDE-6.0-M3/platform7;
/home/rich/.netbeans/dev
  Boot & Ext. Classpath   =
/usr/java/jdk1.6.0/jre/lib/resources.jar:/usr/java/jdk1.6.0/jre/lib/rt.jar:/usr/java/jdk1.6.0/jre/lib/sunrsasign.jar:/usr/java/jdk1.6.0/jre/lib/jsse.jar:/usr/java/jdk1.6.0/jre/lib/jce.jar:/usr/java/jdk1.6.0/jre/lib/charsets.jar:/usr/java/jdk1.6.0/jre/classes:/usr/java/jdk1.6.0/jre/lib/ext/dnsns.jar:/usr/java/jdk1.6.0/jre/lib/ext/sunjce_provider.jar:/usr/java/jdk1.6.0/jre/lib/ext/localedata.jar:/usr/java/jdk1.6.0/jre/lib/ext/sunpkcs11.jar
  Application Classpath   =
/usr/local/netbeans/NetBeans-IDE-6.0-M3/platform7/lib/boot.jar:/usr/local/netbeans/NetBeans-IDE-6.0-M3/platform7/lib/org-openide-modules.jar:/usr/local/netbeans/NetBeans-IDE-6.0-M3/platform7/lib/org-openide-util.jar:/usr/java/jdk1.6.0/lib/dt.jar:/usr/java/jdk1.6.0/lib/tools.jar
  Startup Classpath       =
/usr/local/netbeans/NetBeans-IDE-6.0-M3/platform7/core/core.jar:/usr/local/netbeans/NetBeans-IDE-6.0-M3/platform7/core/org-openide-filesystems.jar:/usr/local/netbeans/NetBeans-IDE-6.0-M3/nb6.0/core/org-netbeans-upgrader.jar:/usr/local/netbeans/NetBeans-IDE-6.0-M3/nb6.0/core/locale/core_nb.jar
-------------------------------------------------------------------------------