Data (even from reputable sources) can contain geometry errors. These can often be tiny geometry errors that are not obvious but that prevent geoprocessing tools from running or producing valid results.

The Check Geometry Validity tool (which can be found by navigating to Vector | Geometry Tools | Check Geometry Validity) takes an input vector layer that is loaded in the canvas and scans the data for errors, such as geometric intersections. The errors can be displayed in a window on the tool or can be output to a point layer. The resulting point layer will have an attribute describing the error. In the following screenshot, you can see that the tool has been run using the DRECP_Alternative1_Integrated.shp sample dataset. This data is portion one of five alternatives for the Desert Renewable Energy Conservation Plan for Southern California. The tool found 267 errors. The errors still need to be repaired, but at least you now know where they are!