bad extract by mask result masking with layer converted from KML

Discussion created by drstrip on Nov 1, 2011
I took a kml file of some regions, converted it to a layer, then tried to use that layer as a clipping mask. The resulting clip is all wrong. Some of the mask interior has no data, and some of the clipped data is outside outside the mask.

Both KML and shapefiles orient polygons using a right-hand rule (most of computational geometry is left-hand rule), so getting this wrong is not likely to be the problem. About all else I can come up with is invisible lines, but why would segments be invisible?