The highlighted line intersects the boundary of the face being modified. To modify a cut face, use open loops that end on the existing boundary without intersecting or overlapping existing edges. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
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