I have a client who is trying to use the Customize window to place the button
"Format - Settings..." onto the toolbar, but it just doesn't appear there
when it is moved. It drags up, but doesn't appear in the toolbar when
dropped.
Other buttons appear fine. When "placed" the modify selection button shows
up and details about the button (name, icon, etc) can be changed; the button
will occassionally appear on the left-hand end of the toolbar but is not
selectable and disappears soon after. Any reason why this would happen?

It happens on his network and my own, so not necessarily company-specific.