That's weird, I can't reproduce it here. I've also tried with 2.28, and that works fine. Any idea about what you are doing different than I do?

This is the dialog that appears after clicking the 'edit' link alongside the users password in the user-admin dialog not the change password dialog produced by 'about me' (2.29.1)

Yeah, that's the one I'm talking about. The latter is an Ubuntu-only change anyway.

Created attachment 151049 [details]
Screenshot of affected dialog

Yes, we're talking about the very same dialog. What happens when you hit tab for the first time, when the first password entry is focused?

Created attachment 151063 [details]
Short Video of event

Hopefully you can see here that focus doesn't move directly between the password and confirmation box

Type in password box, tab once, cursor disappears from password box, focus appears to move to nowhere else, hitting tab for a second time focus is moved to confirmation box and the cursor appears there

Well, thanks for the video, at least it's clear something is going wrong with a hidden widget or something like that. There's a hidden checkbox just below the confirmation entry, it should normally not be a problem, but it's worth a try.

Could you edit the interface file and retry? You should edit the file using:
sudo gedit /usr/share/gnome-system-tools/ui/users.ui

And then search for "user_passwd_quality" to find the lines:
> <object class="GtkCheckButton" id="user_passwd_quality">
> <property name="label" translatable="yes">Check password _quality</property>
> <property name="can_focus">True</property>
> <property name="receives_default">False</property>
> <property name="use_underline">True</property>
> <property name="draw_indicator">True</property>
> </object>

Then you should replace 
> <property name="can_focus">True</property>
with
> <property name="can_focus">False</property>

That may help, in which case I could remove that unused box.

Just in case, what's your distribution, and the precise GTK+ version you're using? Did the problem happen with users-admin 2.28.x? Can you think of any non-standard setting you are using and I'm not (accessibility, keyboard layout...)? Thanks!

No, its not going there, I even added <property name="visible">True</property> so as I could see. But definitely not that one.

For info the distro is Ubuntu Lucid alpha 1 completely unchanged (I see you've already found the bug on launchpad) gtk 2.19.2. No accessibility settings configured, using UK localization (keyboard etc) 

On my karmic machines which are using 2.28.1 gtk 2.18.3 this is not happening and I don't recall it happening on this one while on karmic.

Found it, in the /usr/share/gnome-system-tools/ui/users.ui theres a gtk button behind the confirmation password entry box that can focus its on line 1847 and has id user_settings_chpasswd changing canfocus to false stops the 2xtab keys between password boxes

This is added in the ubuntu src diff by the looks of things so a ubuntu bug not gnome http://launchpadlibrarian.net/37396660/gnome-system-tools_2.29.1-0ubuntu2.diff.gz

For reference, Launchpad bug is https://bugs.edge.launchpad.net/ubuntu/+source/gnome-system-tools/+bug/501976.