I was getting this, even with small surfaces. I've had surfaces at 1280x1600 without any warning from this and simply adding a small, unscaled 128x96 surface, I got this. My level was able to run afterwards, but much later, I got a "lightmap space overrun" error with 16 megabytes used for lightmaps (according to the map compiler anyway). I had over 2000 blocks, mostly small ones flagged as detail. This is my old, abandoned project. My two current projects, however, don't have this issue. My 3D level doesn't use blocks, rather, it uses models instead since they render faster. My other project is nothing more than a bunch of panels which couldn't get lightmaps (or even have to build).