problem fully reproducable:
write in editor 

$${}

and then try to erase by backspace one of dollars ..

java.lang.ArrayIndexOutOfBoundsException: -1
	at org.netbeans.lib.editor.util.GapList.get(GapList.java:459)
	at org.netbeans.lib.editor.util.FlyOffsetGapList.elementOffset(FlyOffsetGapList.java:133)
	at org.netbeans.lib.lexer.EmbeddedTokenList.tokenOffset(EmbeddedTokenList.java:228)
	at org.netbeans.lib.lexer.inc.TokenListUpdater.update(TokenListUpdater.java:218)
	at org.netbeans.lib.lexer.TokenHierarchyUpdate.updateTokenListByModification(TokenHierarchyUpdate.java:338)
	at org.netbeans.lib.lexer.TokenHierarchyUpdate$TLLInfo.update(TokenHierarchyUpdate.java:507)
	at org.netbeans.lib.lexer.TokenHierarchyUpdate.processLevelInfos(TokenHierarchyUpdate.java:314)
	at org.netbeans.lib.lexer.TokenHierarchyUpdate.update(TokenHierarchyUpdate.java:143)
	at org.netbeans.lib.lexer.TokenHierarchyOperation.textModified(TokenHierarchyOperation.java:475)
	at org.netbeans.spi.lexer.TokenHierarchyControl.textModified(TokenHierarchyControl.java:93)
	at org.netbeans.lib.lexer.inc.DocumentInput.removeUpdate(DocumentInput.java:144)
	at org.netbeans.lib.editor.util.swing.PriorityDocumentListenerList.removeUpdate(PriorityDocumentListenerList.java:85)
	at javax.swing.text.AbstractDocument.fireRemoveUpdate(AbstractDocument.java:243)
	at org.netbeans.editor.BaseDocument.fireRemoveUpdate(BaseDocument.java:1401)
	at org.netbeans.editor.BaseDocument.remove(BaseDocument.java:792)
	at org.netbeans.editor.BaseKit$DeleteCharAction.actionPerformed(BaseKit.java:1432)
	at org.netbeans.modules.web.core.syntax.JSPKit$JspDeleteCharAction.actionPerformed(JSPKit.java:597)
	at org.netbeans.editor.BaseAction.actionPerformed(BaseAction.java:290)
	at javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1636)
	at javax.swing.JComponent.processKeyBinding(JComponent.java:2849)
	at javax.swing.JComponent.processKeyBindings(JComponent.java:2884)
	at javax.swing.JComponent.processKeyEvent(JComponent.java:2812)
	at java.awt.Component.processEvent(Component.java:5876)
	at java.awt.Container.processEvent(Container.java:2058)
	at java.awt.Component.dispatchEventImpl(Component.java:4466)
	at java.awt.Container.dispatchEventImpl(Container.java:2116)
	at java.awt.Component.dispatchEvent(Component.java:4296)
	at java.awt.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.java:1848)
	at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:704)
	at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:969)
	at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:841)
	at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:668)
	at java.awt.Component.dispatchEventImpl(Component.java:4338)
	at java.awt.Container.dispatchEventImpl(Container.java:2116)
	at java.awt.Window.dispatchEventImpl(Window.java:2454)
	at java.awt.Component.dispatchEvent(Component.java:4296)
[catch] at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
	at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:104)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:284)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

and then JSP-editor needs to be reopened. Cursor becomes invisible and other strange effects, such as uneditable areas 
and 
java.lang.IllegalStateException: Removing at index=8 but real index is -1 (indexWasMinusOne=true). 
(i can send full trace if needs)
and some other IndexOutOfBoundsException's for positive indexes...


Product Version: NetBeans IDE 6.1 RC1 (Build 200804100130)
Java: 1.6.0_10-beta; Java HotSpot(TM) Client VM 11.0-b11
System: Linux version 2.6.22.9-1mdvcustom running on i386; UTF-8; ru_RU (nb)
Userdir: /home/cy6ergn0m/.netbeans/6.1rc1