I assume that’s a mistake.
I don’t see wpmini-blue.png or wpmini-blue-2x.png in a fresh copy of 3.8.
https://core.trac.www.ads-software.com/browser/branches/3.8/src/wp-includes/images
..and when I grep the entire 3.8 package, the only references to wpmini-blue are the two hits in update-core.php.
Conversely, if you grep the entire package for 3.7.1, you will get references to both the images in /wp-includes/css/admin-bar.css and admin-bar.min.css
So, based on that info, my thoughts are that the references to the images have been removed from /admin-bar.css, and the images have been removed from /wp-includes/images. Seems intentional rather than erroneous.
when I deactivate the UB plugin the PNG shows normally.
Interesting.. What’s the source code say about the path to the image you’re seeing? Maybe the behavior is different in a network (MU) setup? Or maybe I’m just not seeing the entire picture.. which is most definitely possible! It certainly deserves a little more investigation.
[edit] Relevant discussion: https://core.trac.www.ads-software.com/ticket/26036