During Mission 4 of the single-player campaign, you must NOT build a second HQ before you defeat the nearest band of Orks. If you do build a second HQ, it will cause a Fatal Scar Error when you destroy the nearer Ork camp, and you will not be able to meet your victory conditions.

When you destroy the nearer Ork camp, it will prompt you to set up a new base there. After this, you're free to build as many HQ's as you want.

This is a guaranteed way to avoid this error; I was told this by THQ's tech support. If I come across any similar errors in other missions, I'll make sure to mention them here.