This issue was originally marked as duplicate of issue 160431, that is already resolved. This issue is still valid, so this seems to be another issue, but it might be related.

Build: NetBeans IDE Dev (Build 200908022240)
VM: Java HotSpot(TM) Client VM, 14.0-b16, Java(TM) SE Runtime Environment, 1.6.0_14-b08
OS: Windows Vista, 6.0, x86

User Comments:
aethersg: i was editing my drop down menu in nb6.8 when this error happened. i reckon that 6.8m1 still has issues with the visual template as this has happened a few time already.it seems this only occurs when the preview or "design" mode is have problems making the preview for this.

terje: Netbeans 7 Dev 200903250219
 
Editing in an editor window.

java.lang.AssertionError: visnodes.size()=2 snapshot.size()=1
	at org.openide.explorer.view.VisualizerChildren.recomputeIndexes(VisualizerChildren.java:87)
	at org.openide.explorer.view.VisualizerChildren.getIndex(VisualizerChildren.java:153)
	at org.openide.explorer.view.VisualizerNode.getIndex(VisualizerNode.java:291)
	at javax.swing.tree.DefaultTreeModel.getIndexOfChild(DefaultTreeModel.java:142)
	at javax.swing.tree.FixedHeightLayoutCache$FHTreeStateNode.createChildFor(FixedHeightLayoutCache.java:953)
	at javax.swing.tree.FixedHeightLayoutCache.getNodeForPath(FixedHeightLayoutCache.java:701)
	at javax.swing.tree.FixedHeightLayoutCache.ensurePathIsExpanded(FixedHeightLayoutCache.java:639)
	at javax.swing.tree.FixedHeightLayoutCache.setExpandedState(FixedHeightLayoutCache.java:282)
	at javax.swing.plaf.basic.BasicTreeUI.updateExpandedDescendants(BasicTreeUI.java:1648)
	at javax.swing.plaf.basic.BasicTreeUI$Handler.treeExpand

GUEST: Error is throws when i mark text, when i delete text, when i write new text....

GUEST: Comenting out html nodes

GUEST: undoing text writing



Stacktrace: 
java.lang.AssertionError: visnodes.size()=8 snapshot.size()=7
        at org.openide.explorer.view.VisualizerChildren.recomputeIndexes(VisualizerChildren.java:87)
        at org.openide.explorer.view.VisualizerChildren.getIndex(VisualizerChildren.java:153)
        at org.openide.explorer.view.VisualizerNode.getIndex(VisualizerNode.java:289)
        at javax.swing.tree.DefaultTreeModel.getIndexOfChild(DefaultTreeModel.java:142)
        at javax.swing.tree.FixedHeightLayoutCache$FHTreeStateNode.createChildFor(FixedHeightLayoutCache.java:953)
        at javax.swing.tree.FixedHeightLayoutCache.getNodeForPath(FixedHeightLayoutCache.java:701)