More investigation resulted in this correction to calculating the number of labels.

Phil Elson · Phil Elson · commit 7ef723e8346d · 2012-03-01T14:11:13.000Z
diff --git a/examples/pylab_examples/contourf_demo.py b/examples/pylab_examples/contourf_demo.py
@@ -65,7 +65,8 @@
 CS3 = contourf(X, Y, Z, levels,
                         colors = ('r', 'g', 'b'),
                         origin=origin,
-                        extend='both', hatch=['/', '\\'])
+                        extend='both', 
+                        hatches=['/', '\\'])
 # Our data range extends outside the range of levels; make
 # data below the lowest contour level yellow, and above the
 # highest level cyan:
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
@@ -153,7 +153,7 @@ def clabel(self, *args, **kwargs):
         self.rightside_up = kwargs.get('rightside_up', True)
         if len(args) == 0:
             levels = self.levels
-            indices = range(len(levels)-1)
+            indices = range(len(self.cvalues))
         elif len(args) == 1:
             levlabs = list(args[0])
             indices, levels = [], []
