Failed to load the default scheme file
"Failed to load the default scheme file. The map views may be missing some visual elements"

Possible causes
The error is known to occur when trying to start Hammer faster using a shortcut, instead of the Source SDK GUI. Although, it can also happen when openning Hammer from the Source SDK GUI today (Sept 2009).
Possible fixes
- Get SourceScheme.res from one of the GCF files or create a mod.
- Copy it to the ...\SteamApps\username\sourcesdk\bin\resource\ folder, or to the ...\SteamApps\username\half-life 2\hl2\resource\ folder.
If this doesn't work, then you can use Filemon from sysinternal's to find out where Hammer is actually looking for this file.
Another possible fix (for Orange Box):
- Get SourceScheme.res as outlined above
- Copy it to \SteamApps\username\sourcesdk\bin\orangebox\bin\resource\
Yet another possible fix :
It is possible that you have deleted the line
"Tools will load this (ie: source SDK caches) to get things like materials\debug, materials\editor, etc."
or only added "//" before in the GameInfo.txt of your mod or game. This can cause this error or an other when you start HLMV.
- If so, just rewrite the line or erase the "//".
Above Possible fixes need to be... fixed
As there is no such a "resource" folder in today's file structure. The above fixes must be outdated now. Can someone please update these fixes?