Last change tried is the External Compilation / Deprecation switched 
to ON.

After a few project switching (4) the following exception occurs:

java.lang.IllegalStateException: Duplicate pair in treePair1: SL
[/CompilerType/External Compilation pair2: SL[/CompilerType/External 
Compilation index1: -1 index2: -1 item1: 
org.netbeans.modules.java.JavaExternalCompilerType@5b843d item2: 
org.netbeans.modules.java.JavaExternalCompilerType@46d477 id1: 26f29f 
id2: 102c186
	at org.openide.util.lookup.ALPairComparator.compare
(ALPairComparator.java:52)
	at java.util.TreeMap.compare(TreeMap.java:1085)
	at java.util.TreeMap.put(TreeMap.java:463)
	at java.util.TreeSet.add(TreeSet.java:209)
	at org.openide.util.lookup.AbstractLookup$R.initItems
(AbstractLookup.java:788)
	at 
org.openide.util.lookup.AbstractLookup$R.allItemsWithoutBeforeLookup
(AbstractLookup.java:749)
	at org.openide.util.lookup.AbstractLookup$R.allInstances
(AbstractLookup.java:691)
	at org.openide.util.lookup.ProxyLookup$R.computeResult
(ProxyLookup.java:369)
	at org.openide.util.lookup.ProxyLookup$R.allInstances
(ProxyLookup.java:320)
	at org.openide.util.lookup.ProxyLookup$R.computeResult
(ProxyLookup.java:369)
	at org.openide.util.lookup.ProxyLookup$R.allInstances
(ProxyLookup.java:320)
	at org.netbeans.core.Services.services(Services.java:279)
	at org.netbeans.beaninfo.editors.ServiceTypeEditor.updateTags
(ServiceTypeEditor.java:92)
	at org.netbeans.beaninfo.editors.ServiceTypeEditor.getTags
(ServiceTypeEditor.java:148)
	at 
org.openide.explorer.propertysheet.RendererFactory.getRenderer
(RendererFactory.java:138)
	at 
org.openide.explorer.propertysheet.SheetCellRenderer.getTableCellRende
rerComponent(SheetCellRenderer.java:100)
	at 
org.openide.explorer.propertysheet.BaseTable.prepareRenderer
(BaseTable.java:552)
	at 
org.openide.explorer.propertysheet.SheetTable.prepareRenderer
(SheetTable.java:409)
	at javax.swing.plaf.basic.BasicTableUI.paintCell
(BasicTableUI.java:1149)
	at javax.swing.plaf.basic.BasicTableUI.paintCells
(BasicTableUI.java:1051)
	at javax.swing.plaf.basic.BasicTableUI.paint
(BasicTableUI.java:974)
	at javax.swing.plaf.ComponentUI.update(ComponentUI.java:142)
	at javax.swing.JComponent.paintComponent(JComponent.java:541)
	at 
org.openide.explorer.propertysheet.SheetTable.paintComponent
(SheetTable.java:320)
	at javax.swing.JComponent.paint(JComponent.java:808)
	at org.openide.explorer.propertysheet.BaseTable.paint
(BaseTable.java:613)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JViewport.paint(JViewport.java:722)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JSplitPane.paintChildren(JSplitPane.java:1021)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JSplitPane.paintChildren(JSplitPane.java:1021)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintChildren(JComponent.java:647)
	at javax.swing.JSplitPane.paintChildren(JSplitPane.java:1021)
	at javax.swing.JComponent.paint(JComponent.java:817)
	at javax.swing.JComponent.paintWithOffscreenBuffer
(JComponent.java:4787)
	at javax.swing.JComponent.paintDoubleBuffered
(JComponent.java:4740)
	at javax.swing.JComponent._paintImmediately
(JComponent.java:4685)
	at javax.swing.JComponent.paintImmediately
(JComponent.java:4488)
	at javax.swing.RepaintManager.paintDirtyRegions
(RepaintManager.java:410)
[catch] at 
javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run
(SystemEventQueueUtilities.java:117)
	at java.awt.event.InvocationEvent.dispatch
(InvocationEvent.java:178)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:454)
	at java.awt.EventDispatchThread.pumpOneEventForHierarchy
(EventDispatchThread.java:201)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy
(EventDispatchThread.java:151)
	at java.awt.EventDispatchThread.pumpEvents
(EventDispatchThread.java:145)
	at java.awt.EventDispatchThread.pumpEvents
(EventDispatchThread.java:137)
	at java.awt.EventDispatchThread.run
(EventDispatchThread.java:100)

Once the java.lang.IllegalStateException: Duplicate pair in 
treePair1: SL
[/CompilerType/External Compilation pair2: SL[/CompilerType/External 
Compilation index1: -1 index2: -1 item1: 

has occured, the IDE is unusable. Even switching to a project without 
modification to Options, even closing and restarting the IDE.

By unusable I mean that the exception is displayed in a infinite loop.

Reassigning to web module ...

Reassigning to Illya. Adding the NO40 keyword - this issue does not
affect the NB 4.0 release.

I am receiving a similar exception on startup. Exception
(IllegalStateException) is the same, specifics differ from the one
already reported.
Here's the truncated version of mine...
java.lang.IllegalStateException: Duplicate pair in treePair1:
SL[/Browsers/SwingBrowser pair2: SL[/Browsers/SwingBrowser index1: 6 i
ndex2: 6 item1: org.netbeans.core.ui.SwingBrowser@1fb7565 item2:
org.netbeans.core.ui.SwingBrowser@1fb7565 id1: 189d7eb id2: 1d83cf4

In my case, using Alt-F1 to access specific javadocs is unaffected;
however, Shift-F1 to bring up the javadoc search fails with a trio of
exceptions (the IllegalStateException appears again, then an NPE).
Here's the truncated text of the second ISE:
Annotation: Exception during initialization of HTML browser module
java.lang.IllegalStateException: Duplicate pair in treePair1:
SL[/Browsers/SwingBrowser pair2: SL[/Browsers/SwingBrowser index1: 6
index2: 6 item1: org.netbeans.core.ui.SwingBrowser@6c08b2 item2:
org.netbeans.core.ui.SwingBrowser@6c08b2 id1: 1422d0b id2: af17c7

Here's the truncated text of the NPE:
java.lang.NullPointerException
	at org.openide.awt.HtmlBrowser.<init>(HtmlBrowser.java:205)
	at org.openide.awt.HtmlBrowser.<init>(HtmlBrowser.java:155)
	at
org.netbeans.modules.javadoc.search.IndexSearch.<init>(IndexSearch.java:172)
	at
org.netbeans.modules.javadoc.search.IndexSearch.getDefault(IndexSearch.java:612)

If I open the Options window and go to System/System Settings, I
receive the same ISE twice. The Web Browser property reads "Duplicate
pair in treePair1: SL[/Browse..." and is in blue text rather than the
black text of the rest of the properties.
If I click on or in the Web Browser property, I receive the ISE
continuously until I click on the Options window's "Close" button or
click on another property.

I set the Web Browser property to "Swing HTML Browser" almost
immediately after installing 4.0B2 and had made no changes to any
options today, though I had been in the Options dialog earlier in the
day (again, I made no changes, just surfed around).

Software specifics: Windows 2000, Java version 1.4.2_04-b05, NB4.0 Beta 2

Unless there's a better workaround than "delete your NB userdir and
start from scratch", I'd say this definitely merits the P1 priority it
was given and *should* affect the release of a production 4.0. Also,
I'm not familiar with NB source, but this would seem to be a more
widespread problem than a web/jspcompiler problem since I'm not doing
web/JSP development at all.

As for the SwingBrowser, see
http://www.netbeans.org/issues/show_bug.cgi?id=48590

which is already fixed.

Other treepair bugs are here:

http://www.netbeans.org/issues/buglist.cgi?issue_status=RESOLVED&issue_status=VERIFIED&email1=&emailtype1=substring&emailassigned_to1=1&email2=&emailtype2=substring&emailreporter2=1&issueidtype=include&issue_id=&changedin=&votes=&chfieldfrom=&chfieldto=Now&chfieldvalue=&short_desc=treepair&short_desc_type=substring&long_desc=&long_desc_type=substring&issue_file_loc=&issue_file_loc_type=substring&status_whiteboard=&status_whiteboard_type=substring&keywords=&keywords_type=exactlyanywords&field0-0-0=noop&type0-0-0=noop&value0-0-0=&cmdtype=doit&namedcmd=mybugs-web&newqueryname=&order=Reuse+same+sort+as+last+time

My apologies. I wasn't searching through closed/fixed issues when I
was looking for a previous report of my problem or I'd have found it
(and the workaround).

This doesn't happen in 4.0, 4.1.

v