In https://it.wikipedia.org/w/index.php?diff=72025503 , an empty <ref /> tag was inserted. This looks like T63503 but that was fixed, and I can't reproduce the issue myself. In my testing, the reference dialog prevents me from inserting an empty reference, or emptying one, so I don't know what happened in that diff.
In the weekly triage on 2015-04-22, this task was discussed. We considered that it was likely this was a dupe of T96120: "Undo" only undoes Citoid's addition of content, not the whole ref (aka Citoid inserts empty <ref/> tags), and after testing that appears to be the case.