If your getting an error after downloading a map and/or trying to join a non-official map on the server, the fix is fairly simple.
In Windows, go to:
My Computer,
C Drive
Program Files
Steam
SteamApps
counter strike source
maps
In that folder, delete the offending custom maps.
(Don’t worry about losing them. The server will automatically re-download them.)
Re-join the server.
You should be able to get in.
If for some reason you can’t get in or you can’t find that map, do the following:
Click Start -> Find Files and Folders
Do a search for the map name.
(ie, de_russka)
delete the files it finds.
Re-join the server.
I hope this helps sort out some of the problems everyone is having.