Last modified: 2014-02-12 23:35:50 UTC
See example URL. The reason is that the page already existed on labsconsole.wikimedia.org, and that additional revisions with earlier timestamps were imported over the title for the wikitech merge; as with all imports, the older oldids are actually higher and undo doesn't like it.
Well, of course now the URL does something else (equally weird) because I manually undid the edit.