I have verified what Steve posted on Apr 29, 2014. If CSS minification is turned on, the LESS validator does not allow you to save certain file patterns.
For instance, I have a series of LESS files that manage variables, functions, mixins, etc. I use @import (reference) to include these in module LESS files like _footer, _nav, etc. Those compile in the editor just fine. However, if I @import those into a master LESS file called Globals, I get errors. Looking at the request through the Chrome console shows that the modules are coming back minified which seems to cause the LESS compiler to choke. With CSS minification turned off, everything works as expected. I would like to have CSS minification turned on. It would make more sense if the editor never got the minified version regardless of the global setting to avoid these compilation issues.
FYI, you can turn CSS minification off in Kentico 8 at Configuration > Settings > System > Perfomance