The constant value for GL_DECAL is 0x2101 instead of 0x2102. This was tripping up Half-Life when making the water texture transparent when under water. The Half-Life port uses its own OpenGL header, meaning this error wasn't hidden by us.
glTexImage2D
GLMat.cpp
GLMatrix.cpp
glColor4b
glPointSize
glColorMaterial
GL_COLOR_MATERIAL
glIsList
for
GL_POLYGON_OFFSET_FILL