This bug was originally marked as duplicate of bug 223309, that is already resolved. This bug is still valid, so this seems to be another bug, but it might be related.

Build: NetBeans IDE Dev (Build 201301060001)
VM: Java HotSpot(TM) Client VM, 23.6-b04, Java(TM) SE Runtime Environment, 1.7.0_10-ea-b16
OS: Windows 7

User Comments:
wissenstein: Just gave command "Close required projects".

GUEST: I was closing a project.

aldobrucale: Closing a project

mfriedenhagen: Switching to another group.

tboudreau: Has happened twice when switching between an open project group with many maven projects open, and project group "none"

mienamoo: I deleted some source files in one of my open projects and then switched project groups.

mienamoo: Switching project groups

lhochet: with extra -J-Dorg.openide.filesystems.FileUtil.level=FINE, occured when switching between project groups composed of Maven projects

ehsavoie: Reproducing bug #224430

musilt2: Created Maven CRUD demo sample, added all project to free group, created 2 j2se apps, added them to another free group, and was swithich couple times between those two groups

rptmaestro: Opened a project group

pekarna: Switched from current project group (JTexy) to another.

lforet: I was closing maven project

manikantannaren: Opened 3 maven projects
Edited code
Collapsed all three projects
Closed all three projects

bht: Closing a small Maven project

javydreamercsw: Closing multiple projects.

ionuion: renamed maven project (project name, project artifactId and project folder)

fverhoef: I was closing 2 projects. The directory mentioned in the summary had just been added and commited in svn.

GUEST: close maven project

kosmonaFFFt: Close maven project...

sishbi: Reopening a master project

ehsavoie: I can reproduce the bug with this maven project after it has been completely loaded and with other maven projects opened.
If you want to try to reproduce it the project being free is available at https://github.com/Silverpeas/Silverpeas-installer-package

GUEST: Switching groups

musilt2: had Maven CRUD demo sample, as one free project group, had 2 j2se apps as second free group, and was switching couple times between those two groups

aldobrucale: Closing maven project

mienamoo: Switching between project groups

javydreamercsw: Closing 28 Maven projects




Stacktrace: 
java.lang.IllegalArgumentException: Was not listening to Z:\NetBeans\validation-manager\VM-Core\src\main\resources
   at org.openide.filesystems.FileUtil.removeFileChangeListenerImpl(FileUtil.java:328)
   at org.openide.filesystems.FileUtil.removeRecursiveListener(FileUtil.java:437)
   at org.netbeans.modules.maven.cos.CopyResourcesOnSave.closed(CopyResourcesOnSave.java:111)
   at org.netbeans.modules.maven.ProjectOpenedHookImpl.projectClosed(ProjectOpenedHookImpl.java:289)
   at org.netbeans.spi.project.ui.ProjectOpenedHook$1.projectClosed(ProjectOpenedHook.java:89)
   at org.netbeans.spi.project.ui.support.UILookupMergerSupport$OpenHookImpl.projectClosed(UILookupMergerSupport.java:260)