In https://it.wikipedia.org/w/index.php?diff=72454319 , it looks like the user's goal was to remove a few notes in parentheses, but somehow they removed the closing `]]` of a nearby link as well, causing the opening `[[` not to be balanced any more, and thus causing nowikis.
The only explanation I can find at the moment (other than this being a bug) is that the user, for whatever reason, removed the linked text, then re-inserted it manually with the opening brackets but not the closing ones. And they would have had to re-insert that through copy-paste, otherwise the link dialog would have opened when they typed `[[`.
This is far-fetched, so I'm opening this ticket to see if this could have been a bug.
I'm also going to link to https://fr.wikipedia.org/w/index.php?diff=113841450 , in which another internal link was mangled, albeit slightly differently: The resulting wikitext was `[foo]`. This could have been done manually as well, but it also appears unlikely.