I think it has something to do with complex/carved brushes. I got TONS of them in my dm_castle level, especially when doing unintelligent things like, say, carving a sphere in an imperfect half. As far as I can tell, though, it is harmless - I seem to get them in all my maps, and everything still works fine and the performance is as slick as a wet seal pelt (even when doing full compiles).