In MediaWiki.php we have some logic that tries to gracefully handle situations in which only some parameters are passed through query string.
- If a page title is provided and the diff parameter is passed but without a value, it will default to the last diff for that title. For example, https://en.wikipedia.org/w/index.php?title=Foobar&diff= will take you to the latest diff in that page's history
- If a diff revision ID is provided but no title is provided, it will deduce the title from the revision. For example, https://en.wikipedia.org/w/index.php?diff=968976137 works even though it doesn't pass a title parameter.
- If nothing is provided, it default to the main page (which is why https://en.wikipedia.org/w/index.php takes you to the main page)
However, there is one edge case in which it has an expected result (at least IMHO). Going to https://en.wikipedia.org/w/index.php?diff= will take you to the latest diff for that wiki's main page. Instead, it should show a warning that neither the title nor the diff revision ID is provided.