This is a big issue with toposolids and subdivisions. The best I can figure is that the error occurs SOMETIMES when a border for a subdivision overlaps the border of its toposolid. But this does not result in an error every time. I haven't figured what other variables are at play yet.
The best solution I have so far is to offset any subdivision borders slightly away from the border of the toposolid. However, this distant is also not consistent. Generally, I offset it by an inch. But occasionally, this still kicks back the error. Moving it a little further fixes it.
So there is definitely something going on in that area. Its not simply because the borders overlap, because this does not kick back the error all of the time, and the error still happens sometimes when the borders are close but not quite overlapping, though this is significantly less common.
There also appears to be a link to the elevation of nearby points for the toposolid. The errors seem more frequent when points lead to steeper grades.
When drawing a subdivision, I usually do it in chunks. I draw some area of it, finish to confirm there are no areas, edit it to add a bit more, finish to check, etc. This takes time, but its a lot less time than doing a big subdivision, like a parking lot, to completion only to find there is an error somewhere. It can take a long time to find and isolate the problem portion.
