I have several deleted contributions on mediawiki.org in the "Data:" namespace, which no longer exists since T124748. Admins on mediawiki.org can see this at https://www.mediawiki.org/wiki/Special:DeletedContributions/Pppery. These contributions are formatted as `Special:BadTitle/NS486:Title`, which is fine, and clicking on the big red link gives a bad title page, which is also fine.
{F58588132 height=100}
However, if you click "diff" or "view", it gives the following page:
{F58588141 height=100}
This says "The action you have requested is limited to users in the group: Administrators." even though I am an admin, and can see other deletion logs.
Ideally, these URLs would just work, and display whatever deleted revisions are stored in the database at the invalid title (probably not allowing undeletion). Slightly less ideally, it would give an error message explaining that the title is bad, instead of an error message saying I need permissions that I already have.