I am using automatic code-folding (auto-fold everything except nested classes).
I was manually refactoring a class by extracting some methods to separate
classes (subclasses). At one moment, I got an IndexOutOfBoundsException
("Invalid line index=1791 >= lineCount=1791") and since then, the editor became
unusable. I wanted to Cut (Ctrl-X) two methods from one source file and paste it
to the second file. But whenever I marked the block of two methods and pressed
Ctrl-X, I got the IOOBE. I found that I could Cut bodies of the methods (leaving
the method bodies empty) without getting the exception. But then, when I tried
to delete some of the method's curly braces (the goal was to replace
'methodSignature(){...}' with 'abstract methodSignature();'), I got other
exceptions of the same sort and the editor became visually totally broken - e.g.
it displayed several dozens duplicates of a single line across the whole screen.
I rather shut down the IDE and decided to finish the refactoring in ViM.

Here is the begginning of the first exception:
SEVERE [global]
java.lang.IndexOutOfBoundsException: Invalid line index=1791 >= lineCount=1791
        at o.netbeans.editor.LineRootElement.getElement(:92)
        at o.n.editor.DrawEngineDocView.createCustomView(:216)
        at o.n.lib.editor.view.GapBoxView.customReloadChildren(:1838)
        at o.n.editor.DrawEngineDocView.customReloadChildren(:197)
        at o.n.lib.editor.view.GapBoxView.reloadChildren(:1747)
        at o.n.lib.editor.view.GapBoxView.rebuild(:586)
        at o.n.lib.editor.view.GapBoxView.offsetRebuild(:609)
        at o.n.editor.DrawEngineDocView.foldHierarchyChanged(:294)
        ...

--
NetBeans IDE Dev (Build 070223)
1.6.0; Java HotSpot(TM) Client VM 1.6.0-b105
Linux version 2.6.14-1.1656_FC4 running on i386
en_US (nb); UTF-8