There's this lovely feature in SW17, "Shaded Sketch Contours", that's supposed to show you if a sketch has good contours for extrusion or other such features The problem I'm having is that it shows a shaded area (as if I can extrude it), but then I go to rebuild, and I see a series of error messages:

So I use Check Sketch For Feature, and no problem (no gaps, not even small ones):

I tried hiding all of the other bodies (now I'm on a list), checking for floating entities, etc. The contour was partly created by converting an entity (I tried the "wiggle the endpoints" trick), partly through the intersection of the circle and the "8" shape, partly through reference lines to other bits of geometry (tangent to the 8, a point on that other shape, etc).

The odd thing is that the sketch will clip between shaded and not shaded - if I toggle "Shaded Sketch Contours" button off then on when it's not shaded, it will suddenly be shaded.

I might be able to solve by redoing the sketch a third time, or using the sketch to make a new feature instead of rebuilding this one, or even restarting SW, but I'm kind of a curious guy - I'd like to know why this sort of thing happens so that I can avoid it in the future.

-John