WAD Files in res File
Any custom .wad files should be included on a line in the .res file. Most people include the wad file as the first line after the introductory comment line.
It is usually assumed that players and server admins have the Half-life wad and other standard wads that come with Half-life. But that may not always be the case. For example, if a dedicated server was set up by copying the svencoop files over from another installation, it might not have those wad files available.
One thing you can do to troubleshoot a possible problem here is to run the ResGen program, mentioned at the end of this tutorial. If resgen puts some of the HL wad files in your res file, such as liquids.wad or decals.wad, it wouldn't hurt to include them in your res file. At least that way if the player or server doesn't have those files available, an error message would show up saying they are missing.
Information Note: If anyone needs any of the standard HL wad files, you can get them using the following procedure:
To get the half-life wad, for instance, look for the half-life.gcf file. (gcf stands for game cache file). You must extract the wad files out of the gcf file or have the half-life.gfc available in a path above svencoop. Of course if the half-life.gfc file is in the upward path from svencoop you probably don't need the wad files.
To do an extraction get Gcfscape from Nem's tools. Make sure steam is turned off, then point the gcfscape program to the location of half-life.gcf. Once it initializes it will show you the structure starting with /root of the file. Navigate down to the valve folder and right click to extract the folder to a temporary location. Once that is done you can go to that location and find the standard wad files.