Add another example of PEP 479 justification, courtesy of Isaac Schwabacher

pep-0479.txt

@@ -72,9 +72,9 @@ stumble on these cases by accident::
             print('commit')
  
     def do_it():
-        print('Refactored preparations')
+        print('Refactored initial setup')
         yield # Body of with-statement is executed here
-        print('Refactored finalization')
+        print('Refactored finalization of successful transaction')
 
     def gene():
         for i in range(2):
@@ -92,7 +92,20 @@ subtle bug: if the wrapped block raises ``StopIteration``, under the
 current behavior this exception will be swallowed by the context
 manager; and, worse, the finalization is silently skipped!  Similarly
 problematic behavior occurs when an ``asyncio`` coroutine raises
-``StopIteration``, causing it to terminate silently.
+``StopIteration``, causing it to terminate silently, or when ``next``
+is used to take the first result from an iterator that unexpectedly
+turns out to be empty, for example::
+
+    # using the same context manager as above
+    import pathlib
+
+    with transaction():
+        print('commit file {}'.format(
+            # I can never remember what the README extension is
+            next(pathlib.Path('/some/dir').glob('README*'))))
+
+In both cases, the refactoring abstraction of ``yield from`` breaks
+in the presence of bugs in client code.
 
 Additionally, the proposal reduces the difference between list
 comprehensions and generator expressions, preventing surprises such as