See example URL. The reason is that the page already existed on wikitech.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.

Version: 1.21.x
Severity: minor
URL: https://wikitech.wikimedia.org/w/index.php?title=Deployment&action=edit&undoafter=51695&undo=8451