Maps that you do not have appear as darker gray in the server list, but this is case sensitive.

That means, if you have "map_name", but the server hosts "map_Name", it will appear as though you don't have the map.

The problem comes when you join the server, and have to re-download the map.
This differs in Linux, because it would let the two examples exist in the same folder.

TL;DR: Very messy.