Urls of the ?title=ArticleName&diff=123456789 and ?title=ArticleName&oldid=123456789&diff=prev variety (such as from the patrol log) display an error when the referenced revision does not exist: the page title displays as "Error" and the MediaWiki:Difference-missing-revision text is displayed. The page content as of that revision ID still displays.
Example: https://en.wikipedia.org/w/index.php?title=Anglicisation_of_names&diff=prev&oldid=411729326
Compare with older MediaWiki behavior: https://rationalwiki.org/w/index.php?title=George_Tarleton&diff=prev&oldid=1730578