DEVEL: Isolating misbehaving polygons in cartopy's matplotlib interface

Suppose there is a bug that you would like to track down relating to a contour being drawn in matplotlib, but being transformed in cartopy, it is important to be able to isolate the bad polygon in question so that we can target a fix, and ultimately add a test to ensure that the fix continues to be effective through future cartopy versions. This wiki covers the details on how to reduce the scope of the problem so that it is easy to classify and ultimately quicker to fix.

1. The first step is to be able to reproduce the problem consistently, if you can't do that, then there is a much bigger problem for which you should report as much information as possible.