NetBeans IDE Dev (Build 200609171800)
1.6.0-rc; Java HotSpot(TM) Client VM 1.6.0-rc-b98
Windows XP version 5.1 running on x86
cs_CZ (nb); Cp1250

Description:
============
In attachment there is form file. Add this form to your project and in design
mode, take right margin in mouse and try to resize it in direction to the left.
Then an exception will occure.

I found method how to create this issue but I am not sure if it will appear in
every case. That why I enclosed form file.
1) create JFrame and insert two JPanels into it
2)these two panels should cut frame in two parts (north and sourth part)
First one should be extended to span between the container's left and right
margins in accordance with the recommended offset.
The second one is added in the same way but under the first one (you can see
form in attachment). 
3) Select both JPanels and check attribute Horizonatal Resizable.
Buttons "Change horizontaly and verticaly resizability" should not be pushed
4) add JLabel and JTextField into the first Panel. Swings should be in the same
horizontal group with preffered gap between them. JLabel will be placed in the
left corner. The JTextField's right edge snaps into alignment with the JPanel's
recommended edge margin.
5) then insert JButton into the right corner of first JPanel. Then JFrame
becomes wider.
6) Use undo button.
7) Then put the right margin of JFrame in mouse and try to resize it in left
direction. Make it smaller. Then exception will occure.

So first issue is thrown exception
The second one is that undo didn't work correctly. JButton disappeared but the
size of the frame didn't return back.

StackTrace:
===========
java.lang.AssertionError
	at
org.netbeans.modules.form.layoutdesign.support.SwingLayoutBuilder.convertSize(SwingLayoutBuilder.java:297)
	at
org.netbeans.modules.form.layoutdesign.support.SwingLayoutBuilder.fillGroup(SwingLayoutBuilder.java:201)
	at
org.netbeans.modules.form.layoutdesign.support.SwingLayoutBuilder.composeGroup(SwingLayoutBuilder.java:172)