brush with coplanar faces.Example:
Entity 10, Brush 0, Side 5: has a coplanar plane at (-753, -9, 251), texture CA1X_CON1B
Entity 10, Brush 0, Side 6: has a coplanar plane at (-753, -32, 251), texture CA1X_CON1B
To fix this problem, either move the vertex causing the coplanar warning to make the brush convex again, or move the rogue vertex onto one of its neighbors to destroy it. There is a new program HLfix that is supposed to fix these.
This is now your God. Obey it.