Index: src/zope/interface/interface.py
===================================================================
--- src/zope/interface/interface.py	(revision 83232)
+++ src/zope/interface/interface.py	(working copy)
@@ -293,6 +293,10 @@
     def changed(self, originally_changed):
         """We, or something we depend on, have changed
         """
+        try:
+            del self._v_attrs
+        except AttributeError:
+            pass
 
         implied = self._implied
         implied.clear()
Index: src/zope/interface/tests/test_interface.py
===================================================================
--- src/zope/interface/tests/test_interface.py	(revision 83232)
+++ src/zope/interface/tests/test_interface.py	(working copy)
@@ -327,6 +327,29 @@
 
 """
 
+
+def description_cache_management():
+    """
+There was a bug where the cache used by Specification.get() was not
+cleared when the bases were changed.
+
+    >>> class I1(Interface):
+    ...     a = Attribute('a')
+
+    >>> class I2(I1):
+    ...     pass
+
+    >>> class I3(I2):
+    ...     pass
+
+    >>> I3.get('a') is I1.get('a')
+    True
+    >>> I2.__bases__ = (Interface,)
+    >>> I3.get('a') is None
+    True
+    """
+
+
 def test_suite():
     from zope.testing import doctest
     suite = unittest.makeSuite(InterfaceTests)