https://en.wikipedia.org/wiki/Special:ApiSandbox tries to load
https://en.wikipedia.org/wiki/images/ui-bg_glass_75_e6e6e6_1x400.png
The background color for the buttons are missing.
In debug mode the image is loaded correctly:
https://en.wikipedia.org/wiki/Special:ApiSandbox?debug=true loads
https://bits.wikimedia.org/static-1.23wmf9/resources/jquery.ui/themes/default/images/ui-bg_glass_75_e6e6e6_1x400.png
Version: 1.23.0
Severity: minor