1) Deactivate any plugin and restart IDE
2) In plugin manager check that deactivated plugin and any other activated and push uninstall button
3) Push Uninstall button and then choose Restart IDE later
result: following FNFE is thrown:
java.io.FileNotFoundException: D:\install\6.0_200711261600\userdir\update\deactivate\to_uninstall.txt (Access is denied)
	at java.io.FileOutputStream.open(Native Method)
	at java.io.FileOutputStream.<init>(FileOutputStream.java:179)
	at java.io.FileOutputStream.<init>(FileOutputStream.java:131)
	at org.netbeans.modules.autoupdate.services.Utilities.writeMarkedFilesToFile(Utilities.java:308)
	at org.netbeans.modules.autoupdate.services.Utilities.writeFileMarkedForDelete(Utilities.java:278)
        ...
If you look into the plugin manager, those two plugins are marked with the symbol meaning that it needs restart of IDE
to complete deactivation. However, if you restart IDE manually, those two plugins are in the same state as before - one
is activated and the other is not, the uninstall didn't work.
Note that if you would have chosen Restart now in step 3), it works as expected.

Product Version: NetBeans IDE 6.0 (Build 200711261600)
Java: 1.5.0_13; Java HotSpot(TM) Client VM 1.5.0_13-b05
System: Windows Vista version 6.0 running on x86; Cp1252; en_GB (nb)