Entity 10, Brush 0, Side 5: has a coplanar plane at (-753, -9, 251), texture CA1X_CON1BNote: Currently, images are not working in the wiki. Click link to view example image.http://www.slackiller.com/tommy14/coplanar-2.gif
The most common "Coplanar Plane" error is a rectangle with an extra vertex between two corners, like shown in the picture. It's harder to identify these if the "extra" vertex is really close to one of the corners, which can be barely recognizable if the brush is "shattered" or otherwise damaged. (Hexagons and other many-sided objects should also be scrutinized if they are close to the target coordinates the error messaged gives you)
The x and y coordinates the error message gives you will be very close (usually) to where the coplanar brush is, though the z coordinate will be completely erroneous. The entity and brush number it gives you are almost ALWAYS incorrect so ignore them.
Besides the coordinates, the coplanar plane error message gives you the texture of the offender. Knowing this, you can texture the area of the coordiantes with a "standalone" texture or anything different from the texture set of your current map. In this way, you can narrow down what brushes to look for and know if you're getting close if your "standalne" texture comes up in the error message.
Another way to immediately identify a "coplanar" solid is when you try to clip a brush that is "coplanar", the clipping comes out all weird!! (when you slice a line through the object, instead of cutting it in half, it will "portion" it in to very strange quantities, or leave parts of the brush unnclipped, that should be included.
A final but important note on coplanars: A single leaf-saw error can cause completely erroneous coplanar areas, so if you're having a problem with the coplanar plane error and you can't find the problem, check your compile log for some leaf-saw errors as well as a possible source.