I just wanted to test MediaWiki:Group-sysop.js at beta.wmflabs.org. At first it didn't seem to work so I switched to &debug=true mode to be able to set linewise breakpoints etc.
Surprise, surprise: It didn't work in debug mode, even if the code was correct. It took some time until I figured out what the problem was:
The callback function of mw.loader.using() was called when the script arrived. But in debug mode it's only a script that implements modules by pointing to their indivial URLs, to load the modules separately. Even though the functionality (jQuery.byteLimit in my case) was not available yet, it got used - and hence my script broke.
Without debug mode it works :-(