Did you make the entire brush a displacement??
No, just the visible sides. It goes away when I don't use use -fast so I'm not worried about it as a problem, I just think it looks cool enough to post a screenshot of it.
Plus I guess it's a nice visualization of my in-progress displacement setup.
edit - the top and sides are the same brush, I never saw a good reason to split them into two separate brushes, just seems to make more visual noise in Hammer.