1. Do not include portions of the boundary when constructing curves for trimming. Trimming curves may cross or end on boundaries.
2. Avoid cutting through the degenerate boundary a surface such as the pole of a sphere or surface of revolution.
3. Avoid cutting along (across is OK) a seam of a surface. A seam exists, for example, at the left/right boundary of a cylinder. You can detect seams with the curve extraction dialog box, and then canceling out after you know where the seam is. Avoid cutting a seam rotating the object because many objects with seams are symmetrical.