On two separate web hosts. Checked with versions of IE, Chrome, Firefox, Opera and various versions of Windows, from XP to 8 with PCs on different networks/Internet connections. MediaWiki 1.21.2 on Apache 2.2, with PHP 5.3.27 and both MySQL 5.5.32 or 5.3.12. It's the same with Vector and other skins.
I've installed Extension:Gadgets and can get other gadgets, e.g. mySandbox or DotsSyntaxHighlighter to function as expected. I've tried both hotlinking to and copying from commons.
In Chrome's console there are no errors.
In the page's head section there is:
<script async="" src="//commons.wikimedia.org/w/index.php?title=MediaWiki:Gadget-HotCat.js&action=raw&ctype=text/javascript"></script> but in the categories line there is nothing from HotCat.
Version: REL1_21-branch
Severity: normal