A tough one to figure out, vague error that usually shows up when you start the game or during compile, or even when WC/Hammer starts up - you have gone over the memory limit somewhere for some reason. It can be too little RAM (128M is about minimum), a leaf saw into leaf error, too long pathnames, too many textures, too big a level, too big or too many model/sprites, too big a wav sound file - or it could be that old "too many wads" mistake, a huge "noob" brush around the map to prevent leaks, too many SKY faces on hidden brush faces in the level - or even something else. Example: In early versions of NS, the 3D area info location entities were a major cause.
If it happens during compile, do not use WC/Hammer to "run" the map, but use a front end or batch file to compile with. You could also get more RAM.
(The following may still be handy, however be aware that Software mode allocates memory dynamicly, so it may automaticly seem to "fix" any error. Therefore the following info MAY lead you into a dead end:
Some brush-based entities have rendering properties that, when they interact in OpenGL/D3D video cards, they crash. Switch Half-Life to Software mode first, then load the map. If it loads you now know it is something that is handle different in software, then for Opengl/d3d check water, glows, additive, sprites, transparencies (illusionaries, windows, ect.), env_beams and so on.)
If it happens in opening a level in WC, it may be you have an animating model showing in one view. You must switch to single view instead of multiple, and try different ones until you get past it.