1. A method for isolating failed routing resources on a programmable integrated circuit, the method comprising: receiving a set of failed test patterns that generated erroneous results when applied to a set of failed test paths, the failed test paths connecting together routing resources on the programmable integrated circuit; identifying a subset of the routing resources that occur most frequently in the failed test paths; and generating new test patterns including new test paths for the subset of the routing resources that occurred most frequently in the failed test paths.