It was mentioned today during workshop in Berlin from one of attendees. But I was already thinking about similar before.
The story is that usually if you developing with Magnolia and especially if you discover any issue on production and going to check it, first on prod, then trying to reproduce on local and maybe later on test server. Then in one day, maybe in one hour you may found yourself to having opened production author, public1, public2, test-author, test-public, local author, local public, etc... at same time, all of them looks absolutely same except url in browsed address bar all looks same and its really easy to do mistake and remove or edit something on wrong instance because of that.
So the idea is to have for example optional string, or box, or colored box, or colored background, or "prefix" around Magnolia logo, etc... which may be defined in properties file, because you may have different property file for each webapp setup.
I attached screenshot of what I can imagine works well in case I may have property in magnolia.properties like:
magnolia.instance.label="<span style='color:#fff'>www.mgnl.com - localhost</span>"