In attempt to make a ramp, I've made a brush with two triangular faces. Because I was going to convert it to an displacement, so in attempt to be smart I've modified it to a quadrangle with three verticies in one line huh. Everything was fine, until compile time. Hammer detected the triangular displacements, deleted them and refused compiling. Hammer still gives an error when trying to compile, although brush with ID given by compiler doesn't even exist. And now my map flickers in the 3D view like mad horse$#!+, I also see $#!+ and I can do $#!+. However, when I turn visibility of the Displacement visgroup off, flickering stops. Like this, but with more variety of effects and it changes everytime I move the camera.