The core reprojection method (Shapefile.ReprojectCore) will iterate all shapes in a Shapefile, transforming as many as it can, bypassing shapes that cannot be transformed, and maintaining a count of how many were successfully transformed.
If the map has to reproject a layer while adding it to the map (via MapView.ReprojectLayer), a call is made into the reprojection method, and the number of reprojected shapes is returned. This number is compared with the total number of shapes in the file, and if they differ, the process fails and the layer will not be added to map.
There are times and circumstances in which you might want layers to be added to the map even if some of the shapes could not be transformed. To provide for this, a new Global Setting will be added, called AllowLayersWithIncompleteReprojection. The default value will be false, so that existing behavior is not changed, but when set to true, will allow layers to be added to the map even if not all shapes could be transformed.