The undo/redo menu options work only in the editor pane. So, if one deletes a property in the verification pane, then it cannot be recovered by the undo option.
Another aspect of this bug is that if one choses undo while in one of the other panes, it still has effect on the editor pane.
A simple fix would be to turn this part off and grey the menu points while the other panes are active.
Relates to bug 317
As this is a minor usability issue, I reduce priority and delay the work to post 4.0.
This has been fixed in development version (trunk: 4.1 branch) for quite a while now (by disabling the menu/toolbar actions).
It would be nice to have undo/redo within verifier though, but it will have to wait until properties become part of the "project" document, i.e. when we will have all elements in one file (or in one DOM representation at least).