I was typing a reply for
https://www.mediawiki.org/wiki/Topic:Szxfw53ct6fyj35v
and after I wrote incorrectly the name of the extension I decided to just copy and paste "ContentTranslation" from another tab where this text was visible. To my surprise, when I double clicked on the word I had written and pasted the right string, I got the following JS error:
{ "errorMessage": "TypeError: cur is null", "url": "https://www.mediawiki.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=vector&version=m7F5ikBY", "lineNumber": 66, "columnNumber": 316, "errorObject": {} }