Find Visible Detail Sides...Bad detail brush side

Find Visible Detail Sides...Bad detail brush side

This error is mostly related to using multiple incompatible tool textures on the same brush. Aside from Hint/Skip, Occluder/Skip and few others, you cannot mix 2 or more tool textures (such as hint/block or clip/areaportal for example) on the same brush. Your best bet is to hide all visgroups in Hammer and keep only the "tool brushes" visgroups and search for any brush that has incompatible tool textures. Another (less efficient) choice you got is to open Hammer's texture browser, type "tool" in filter field, then tick the "only used textures" option, then click on "mark" for every tool texture you used to mark the solids in the map so you can easily inspect them. Another thing is that you are compiling on fast vvis. Get in the habit on not doing so since it will conceal errors in the map and will generate bugs and artifacts that can be avoided when compiling on normal vis. Hope this helps Will2k
This was taken from Gamebanana.  The brushes that gave me this error was someone putting tree textures on player clip. The map wouldn't compile and threw that error every time.